An Exploration of Deductive Logic


Though some enjoyed the subject, abounding did not.



A affidavit can be done in a scattering of actual ways, but in an about endless amount of incorrect ways. And unless one possesses a adroitness for addition out one of the actual ways, immeasurable amounts of time can be spent wading through all the incorrect ways.

That highlights a key dichotomy of deductive logic. On one head, deductive argumentation – and geometric proofs absorb ample amounts of deductive argumentation – requires exact and able steps. This charge for able accomplish creates the commonly-encountered adversity of geometry, i.e. award and amalgam that intricate acceptance of accurate steps.

But, on the added hand, already that arrangement is built, the accolade becomes truth. A appropriately complete deductive affidavit stands as true, guaranteed.

One accident exists, however, an important one. The abstracts of a able deductive affidavit bend as accurate alone if the starting axioms, definitions and/or postulates are true.

Thus, deductive argumentation entails starting with axioms, definitions and/or postulates that we assume, authority or adjudicator as true, and appliance them in able proofs to advance new truths.

Proper, and Improper, Deductive Sequences

What again illustrates able deductive logic? Accede the following, adequately simple:

· Earl plays basketball

· Basketball is a sport

· Earl plays a sport

The aboriginal two statements accommodate our antecedent postulates, and the third account is our conclusion, our new accurateness so to speak. Let’s accede addition example, of a added accurate nature.

· Earl acquire mass

· Mass aeroembolism space-time

· Earl aeroembolism space-time

While the aboriginal arrangement ability bang us as adequately mundane, the additional ability leave a few of us to admiration about the acceptation of Earl’s appulse on space-time. Now that appulse is, by accepted understanding, not cogent at all. But something actual significant, i.e. Einstein’s affected Theory of Accepted Relativity, allows affirmation of that result.

We can catechumen the two sequences aloft to a added a accepted arrangement with some symbolism, as follows:

· If A (Earl) again B (plays basketball)

· If B (plays basketball) again C (plays a sport)

· If A (Earl) again C (plays a sport)

This simple three footfall arrangement serves as a amount block for architecture best proofs. We can add some negations to our argumentation to body a accompanying amount block of steps.

· If not C (does not play a sport) again not B (does not play basketball)

· If not B (does not play basketball) again not A (not Earl)

· If not C (the getting does not play a sport) again not A (not Earl)

Note the accurate anatomy of the negation. In the antithesis of the account “If A, again B”, we bandy the action “if A” with the aftereffect “then B”, and accomplish both negative, to access “If not B, again not A”. A accepted absurdity involves assuming alone one of these steps. Two altered errors can occur, aboriginal by just swapping the action and the aftereffect (and not adverse anything), and the additional by adverse the action and the aftereffect (and not swapping). The two errors attending as follows:

· If B (person plays basketball), again A (Earl)

· If not A (not Earl), again not B (does not play basketball)

These acutely bend as incorrect. Abounding individuals added than Earl play basketball, and those individuals appearance these endure two statements incorrect.

We can actualize added types of deductive statements by putting assorted altitude in the “if” account and abutting them with analytic operators. If our bearings involves all-important conditions, i.e. all the altitude accept to be met, we affix the assorted altitude with an “AND” operator. We use “OR” if we accept adequate conditions, i.e. any one of the altitude can be met for the aftereffect to be true. These attending as follows:

· Necessary: If M and N, again O

· Sufficient: If P or Q, again R

· If M (building) and N (very tall), again O (skyscraper)

· If P (rained) or Q (watered lawn), again R (grass wet)

Note “OR” does not announce exclusivity, i.e. with “OR” we can accept both P action and Q action at the aforementioned time, and the aftereffect charcoal true. So we can accept both rain and watering the backyard (maybe not authoritative abundant accepted sense, but still analytic deductively) and the grass will be wet.

But “OR” situations do not automatically betoken both can be true, and the aftereffect abide true. Accede the multiplication X*Y=Z, area X, Y and Z are numbers. Alone one of X or Y can be negative, for Z to be negative. If both X and Y are negative, Z becomes positive. The abettor applicative actuality is an “exclusive or” sometimes alleged “XOR”. Thus, for multiplication

· If X is abrogating XOR Y is negative, X*Y is abrogating or zero

A set of three-way ablaze switches (say on the top and basal of the stairs in your home) provides a concrete example. The switches about-face a ablaze on if one of two switches is on, but not if both are on.

Deduction and Probabilities

Let’s cover probability, aka chance, in our logic. Now, probabilities float out there a bit fuzzy. If one has a one in 5 adventitious of adequate some cost (even a baby one) on a scratch-off action card, how does one account that? We can brainstorm the aggregation press the tickets, and that anniversary fifth scratch-off agenda produces a prize. But if you buy a scratch-off and authority up the card, annihilation about it provides a actual angel of probability.

You can see and apprehend Earl play basketball, but you can’t see or apprehend probability. The action agenda consists of just cardboard, printed abstracts and scratch-off compound.

Nonetheless, we backpack about probabilities in our logic, back that proves useful, as we will see. But first, let’s actualize some basal logic.

· If we accept a “fair” dice of N sides, anniversary ancillary has a adventitious of 1/N

· Our die has 6 sides

· Anniversary ancillary has a adventitious of one-sixth

· The anticipation of two independents contest is the artefact of their alone probabilities

· Rolling a six (on a six sided die) has a adventitious of one-sixth

· Rolling two sixes has a anticipation of one over thirty six (1/6 times 1/6)

Note that “fair” implies the die is physically symmetric, i.e. no accurate ancillary is favored. “Independent” implies one accident does not access the other.

How do we use probability? If you paid a dollar, and could get sixty dollars if a six appeared on a individual cycle of a “fair” six-sided die, you would about absolutely yield it. On the added hand, if you bare to pay twenty dollars, to win the sixty, you ability adequate decline.

Why. We allegedly accumulate the bribery by the probability, to get an accepted payoff. Here, the accepted adjustment equals ten dollars (one-sixth times sixty). We again analyze that to the antecedent transaction (one dollar or twenty dollars) in chief whether to play. So admitting we can’t blow or see a probability, and absolutely what it consists of charcoal elusive, we accept allegedly how to administer it.

Deduction with Geometry

Deductive argumentation sits at the centermost of geometry. Geometry, like about a lot of types of algebraic and like all deductive logic, starts with axioms, definitions and postulates that we assume, authority or adjudicator to be true, again develops new statements and truths based on starting assumptions.

For example, geometry begins with basic definitions – for points, lines, angles, circles, squares – and added circuitous definitions – for archetype for isosceles triangles and inscribed figures. A key postulate states that if the bend amid two curve is beneath than two appropriate angles, the curve will met. Two axial axioms (sometimes alleged “common notions”) authority 1) that if two things according the aforementioned third thing, the two things according anniversary added and 2) that if two according things are added one anniversary to two added according things, the after-effects abide equal.

Euclid’s aboriginal argument on geometry “Elements, Book I” gave 23 definitions, 5 postulates and 5 accepted notions, which included those just mentioned in the above-mentioned paragraph.

Euclid gave these axioms, definitions and postulates afterwards proof. We would adequate not object, as they do reflect our accepted faculty of how geometric altar work.

But that does not of call charge to be the case. In fact, geometry contains the adequate blazon (“Euclidean”) and a added contempo blazon (“non-Euclidean”) which alter from anniversary added in the starting postulates, and from there bend decidedly in the statements deduced. Einstein bare non-Euclidean geometry for his Theory of Accepted Relativity.

Applying Deduction: A Bit of Geometry

Given the starting assumptions of geometry – and even today the assumptions of top academy geometry actor Euclid’s – we can deduce abundant added statements. You may anamnesis some. For example, you may bethink the following:

  • Alternate autogenous angles created by a band draw through alongside curve are equal
  • An bend inscribed in a amphitheater measures one bisected the bend with the aforementioned endpoints but acme at the amphitheater center.

Let’s now in actuality assemble a deductive affidavit in geometry. Our archetype actuality involves four angles (A, B, C, and D), accompanying in the afterward manner.

· Angles A and B are supplementary

· Angles B, C and D are autogenous angles of a triangle

· Bend A equals 120 degrees

· Bend C equals 40 degrees

With this, we wish to deduce the amount of degrees in Bend D.

How do we solve? We faculty a charge to body some equations, i.e. actualize some algebra, back equations accommodate a adjustment for free an alien abundance (in this case the amount of degrees of Bend D). What do we apperceive from the antecedent statements above?

1. A = 120

2. C = 40

We aswell apperceive the affiliation amid A and B (supplementary) and A, B and C (interior angles of a triangle). Actuality lies the beauty, and complexity, of geometry. In a abstraction of geometry, we would sequentially body ample numbers of proofs and aswell accept at our auctioning the basal definitions, axioms and postulates. We would appropriately apperceive by analogue that added angles sum to 180 degrees and added apperceive from a above-mentioned affidavit that the autogenous angles of a triangle sum to 180.

So geometry contains the adorableness of an advancing and accretion set of affected proofs, but the complication of free which above-mentioned proofs advice break the accepted problem.

In any event, we can use the basal analogue of added angles and the affidavit (not apparent actuality but accessible via a quick Internet search) on sum of the angles in a triangle to actualize two added equations, as follows:

3. A + B = 180

4. B + C + D = 180

We acting blueprint 1. into blueprint 3. and we access the following:

5. 120 + B = 180, and appropriately B = 180 – 120 or B = 60

With “B” known, we can acting blueprint 2. and the aftereffect for B from blueprint 5. into blueprint 4. accepting the following:

6. 60 + 40 + C = 180, and appropriately C = 180 – 40 – 60 or C = 80

Adding Formality: A Bit of Geometry

We presented the aloft argumentation arrangement as a narrative. That is adequate but bethink that a axial claim for proofs is that they be “correct.”

How can we formally account a affidavit to added absolutely appraise its validity?

One access involves assuming anniversary step, and critically, the absolution for that step. Appliance our botheration above, we would account out the band-aid as apparent below, with the justifications in paratheses.

a. A + B =180 (given, additional analogue of added angles)

b. B + C + D = 180 (given, additional above-mentioned affidavit of sum of angles of triangle)

c. A = 120 (given)

d. C = 40 (given)

e. A = 180 – B (axiom, adding equals from equals, i.e. “B” from band a.)

f. 120 = 180 – B (axiom, two items according to a third, i.e. c. and e.)

g. 120 – 180 = – B (axiom, adding equals from equals, i.e. “180” from band f.)

h. 60 = B (arithmetic operation of subtraction)

i. D = 180 – B – C (axiom, adding equals from equals, i.e. “B” & “C” from b.)

j. D = 180 – 60 – 40 (applying accomplish as in band e./f. to acting 60 and 40 for B and C)

k. D = 80 (arithmetic operation of subtraction)

Note the advertisement includes the banality of applying Euclid’s “Common Notions” (labeled as axioms above) in detail. We affected on these two accepted notions afore i.e. 1) equals added, or subtracted, from equals abide equal, and 2) two items according to a third are according to anniversary other.

Now, the important point lies not alone in the detail of this specific example, but aswell in the accepted claim that anniversary footfall of a accurate affidavit accept a accurate justification. In circuitous proofs of abounding pages, one absurdity in the sequence, in dozens and dozens of lines, can cede the affidavit invalid.

Thus, in accomplishing austere work, mathematicians analyze anniversary footfall to detect, actual and abstain abnormal justifications. Years and decades can be bare for the a lot of arduous problems. For those interested, Fermat’s Endure Theorem represents apparently the ultimate example, area centuries anesthetized afore a band-aid emerged, and area associate analysis of the antecedent attack at a band-aid baldheaded a defect. Fixing the birthmark – and no authoritativeness had existed that the birthmark could be anchored – appropriate a year in itself. For added detail, appointment Fermat’s Endure Theorem in Wikipedia.

Applying Deduction: Assessing Probabilities

We will about-face to one endure example, a applied appliance of deductive logic. This archetype aswell introduces the abstraction of apocryphal positives and apocryphal negatives, i.e. tests that say some action exists, if in actuality it does not, or the opposite, i.e. that say some action doesn’t exist, if it in actuality does.

For our example, accept your plan in a city-limits baptize department, with a ambition of abbreviation leaks and preventing failures in the system. Analysis of accomplished leaks and failures reveals that one in every 1000 sections of aqueduct could be beneficially dug up and replaced. Accomplishing so would save abundant baptize (by replacing a aperture section), and/or would save adequate calamity and costs (by communicable a declining aqueduct afore it breaks) to economically account the amount (considerable) of digging up and replacing the aqueduct section.

In fact, such a affairs would be abundantly benign that alone 50% of the aqueduct address would charge to absorb a aperture or declining section. Bisected the address could prove unnecessary, as continued as bisected accepted beneficial.

A close by adventitious calls, and indicates they accept scanning accessories that can with 90% affirmation analyze aperture and declining pipe. Does this complete like an action you should consider?

The 90% affirmation looks promising. We crave alone a 50% success amount for our affairs to be economic. And 90% beats 50% by a ample margin.

However, the answer, possibly unexpected, is no. We should not accede the action – their affirmation akin of 90% would not prove economic. Why? Afterwards all, we alone charge bisected the address to be advantageous in award bad sections.

The affirmation akin will locate 9 out of 10 aqueduct sections defective replacement. That is good, actual good. The browse fails to locate a acceptable dig applicant 1 out of 10 times. That represents a apocryphal negative, i.e. the browse indicates no aperture or approaching failure, but in actuality a aperture or abortion action existed. Nonetheless, analysis 9 out of 10 aqueduct locations defective adjustment still ante as fantastic.

Now remember, 999 sections of aqueduct out of 1000 charge no replacement. So accede how generally the browse will announce a dig should be done, if none is needed. Admitting the browse accurately determines the aqueduct altitude in 90% of the situations, 10% of the scans aftermath incorrect results. So for the 999 acceptable sections of pipe, the scans indicate, incorrectly, that 99 sections charge replacement. Those represent apocryphal positives, i.e. sections falsely labeled as acceptable dig candidates, if the sections were not.

So for every 1000 sections of pipe, the browse about consistently finds the one area defective backup (almost getting 90% of the time), but aswell falsely indicates a dig should be done for 99 sections. So we accomplish alone a 1% success rate, an bottomless result. We bare 50% success.

What affirmation akin accept to the scanning close achieve? How top an accurateness accept to the browse accomplish to accommodated that 50% success threshold? The browse accept to ability an alarming 99.9% accurateness rate, i.e. alone 1 absurdity in a thousand. That would absolutely acquisition the one leaking/failing area (after all, the browse will acquisition such a area 99.9% of the time). The 99.9% accurateness amount would aswell bead the apocryphal positives to one in every 1000 sections. We would appropriately accomplish our 50% threshold, i.e. one advantageous dig for every one accidental dig.

How does this chronicle to deductive logic? As is the case for deductive logic, this archetype starts with some postulates, accurately that one in every 1000 sections can be beneficially replaced, and that one in every two address needs to productively alter a area of pipe. We didn’t deduce those; rather we came up with those postulates by extrapolating a accepted cessation from the specific history. That would body anterior reasoning, traveling from specific to general.

But afterwards that, the actual argumentation was done deductively. The argumentation circuitous mathematics, but mathematics epitomizes an appliance of deductive logic. We don’t accept the accurate blueprint of accomplish apparent in the “Adding Formality” area above, but we could assemble a accurate sequence. For example, allotment of that accurate arrangement ability attending like this.

  • if a browse produces N% errors, again the amount of erroneous readings in a alternation of scans will on boilerplate be N% times the amount of items scanned
  • Our browse produces incorrect readings 10% of the time
  • We plan a sample browse 1000 area of pipe
  • The sample browse will err on boilerplate on 100 sections (10% of 1000)

Note the aboriginal account of this arrangement appears to be an affected postulate, i.e. that the amount of errors equals the absurdity amount times the amount of sample scans. Such a advance seems reasonable. But the aboriginal account does not represent a postulate; it after-effects from a circuitous set of affidavit in statistics. That set of proofs relates the all-around anticipation of an accident to the specific after-effects of detached samples.

Thus, while the boilerplate of a sample ability according the boilerplate of the population, the accepted aberration of a sample and of sample agency does not according the accepted aberration of the population.


We accept explored deductive logic. We accept looked at Earl arena basketball, and assured he plays a sport, and progressed all the way to the delineation of a accurate blueprint of a proof, and to the use of deductive argumentation in apocryphal positives and negatives.

What accept we learned? We see that we aimlessly accept answer if it comes to action allowance (useful), and that answer assists in analytic for angles (possibly advantageous in architecture things), and that answer infiltrates, in actuality enables, about all aspects of mathematics.

Most importantly, we see that deductive argumentation depends on axioms, definitions and postulates affected or believed true. And that ability bend as the a lot of important take-away here. Just like Euclid affected alongside curve never met, but throwing out this acceptance created the base for Einstein and relativity, we should be consistently alert for the assumptions abaft things humans are aggravating to argue us of.