Deductive reasoning is the one that takes premises for granted. I never do it. Therefore I never do deductive reasoning.

Well, enough jokes. It is safe to assume that deductive reasoning never should be used. Arithmetics are invented from induction. Books for children have visual examples how addition and multiplication laws work. I am not saying that induction itself is enough. People give names, have intentions, act and so on. Deductive reasoning follows from absolute awareness in own intentions and desires. And it can be intuitive to think that own desires are true. But desires are not truth-apt for me. Validity and soundness does not apply to them. And one may ask how do I claim this all. Because I want to. I can't be wrong here. Not until I mix them with inductive arguments.

So, deductive reasoning is unsound and fallacious to me. Triangles have three angles because I have an intention to call objects with three angles "triangles". But how do I then talk to philosophers who think that deductive reasoning is meaningful? I think majority of them do so.

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    Life is full of non-deductive "reasoning" : inductive, persuasion (rethoric), authority, faith, advertising. – Mauro ALLEGRANZA Sep 14 at 12:11
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    "Arithmetics are invented from induction. " ???? Very very very debatable. – Mauro ALLEGRANZA Sep 14 at 13:33
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    "ZFC uses laws of logic which are probably exclusive for our finite world and makes a pure speculation how would they work in unlimited world." In a certain sense I agree : Math is "abstract"/"ideal" exasctly because "it uses laws of logic which are probably exclusive for our finite world and makes [the assumption that] they work in an infinite world." – Mauro ALLEGRANZA Sep 14 at 13:35
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    Deductive reasoning was used to design and build the computer you are now using. Is that not "meaningful" enough for you? – Dan Christensen Sep 14 at 13:54
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    You use deductive reasoning right at the start of this post (the part with therefore). – Eliran H Sep 14 at 22:01
up vote 2 down vote accepted

A conclusion is sound (true) or unsound (false), depending on the truth of the original premises (for any premise may be true or false).

At the same time, independent of the truth or falsity of the premises, the deductive inference itself (the process of "connecting the dots" from premise to conclusion) is either valid or invalid. The inferential process can be valid even if the premise is false:

*There is no such thing as drought in the West.

California is in the West.

California need never make plans to deal with a drought*.

In the example above, though the inferential process itself is valid, the conclusion is false because the premise, There is no such thing as drought in the West, is false.

A syllogism yields a false conclusion if either of its propositions is false. A syllogism like this is particularly insidious because it looks so very logical–it is, in fact, logical.

But whether in error or malice, if either of the propositions above is wrong, then a policy decision based upon it (California need never make plans to deal with a drought) probably would fail to serve the public interest.

Assuming the propositions are sound, the rather stern logic of deductive reasoning can give you absolutely certain conclusions.

However, deductive reasoning cannot really increase human knowledge (it is nonampliative) because the conclusions yielded by deductive reasoning are tautologies - statements that are contained within the premises and virtually self-evident.

Therefore, while with the deductive reasoning we can make observations and expand implications, we cannot make predictions about future or otherwise non-observed phenomena.

Let us juxtapose the deductive reasoning process with "Inductive Reasoning"

Inductive reasoning begins with observations that are specific and limited in scope, and proceeds to a generalized conclusion that is likely, but not certain, in light of accumulated evidence.

one could say that inductive reasoning moves from the specific to the general.

Much scientific research is carried out by the inductive method: gathering evidence, seeking patterns, and forming a hypothesis or theory to explain what is seen.

Conclusions reached by the inductive method are not logical necessities; no amount of inductive evidence guarantees the conclusion.

This is because there is no way to know that all the possible evidence has been gathered, and that there exists no further bit of unobserved evidence that might invalidate my hypothesis.

Thus, while the newspapers might report the conclusions of scientific research as absolutes, scientific literature itself uses more cautious language, the language of inductively reached, probable conclusions:

Because inductive conclusions are not logical necessities, inductive arguments are not simply true. Rather, they are cogent:

that is, the evidence seems complete, relevant, and generally convincing, and the conclusion is therefore probably true. Nor are inductive arguments simply false; rather, they are not cogent.

It is an important difference from deductive reasoning that, while inductive reasoning cannot yield an absolutely certain conclusion, it can actually increase human knowledge (it is ampliative). It can make predictions about future events or as-yet unobserved phenomena.

For example:

Albert Einstein observed the movement of a pocket compass when he was five years old and became fascinated with the idea that something invisible in the space around the compass needle was causing it to move.

This observation, combined with additional observations (of moving trains, for example) and the results of logical and mathematical tools (deduction), resulted in a rule that fit his observations and could predict events that were as yet unobserved.

Thereby the deductive process was used as an additional tool, (when it can be used efficiently) but the new knowledge were constructed from a process of induction...


  • It is possible for a syllogism to yield a (seemingly) true conclusion with false premises: 1. Every man is Socrates. 2. Socrates is mortal. 3. Therefore every man is mortal. – rus9384 Sep 15 at 10:40
  • @rus9384-yes, Validity is a guarantee of a true conclusion when the premises are true but offers no guarantee when the premises are false. False premises can lead to either a true or a false conclusion even in a valid argument. – drvrm Sep 15 at 14:21
  • @rus9384 What would your rejection be if one used true premises in a syllogism? Philosophers use the term SOUND argument to arguments that HAS TRUE premises and the conclusion MUST also be true. Deductive reasoning is pretty much built into many species on Earth and not just human beings. If you use tools wrong how can you blame the tools? – Logikal Sep 15 at 14:23
  • @Logikal, I have a strong feeling that deduction (at least partially) is learned, like language or arithmetics. – rus9384 Sep 15 at 14:26
  • @rus9384 , You might be correct in the allegedly BETTER SCHOOL systems. I can tell you for certain all people don’t learn deductive reasoning from math. Other subjects don’t teach the subject effectively and leave too much out. Philosophy teaches deductive reasoning quite differently but I suppose the instructor intentions matter while teaching. Some people do get the short end of the stick as far as information. – Logikal Sep 15 at 14:30

It seems you don't understand the terms. The essence of deductive reasoning:

If all premises are true, the terms are clear, and the rules of deductive logic are followed, then the conclusion reached is necessarily true.

Inductive reasoning, on the other hand:

Inductive reasoning is a method of reasoning in which the premises are viewed as supplying some evidence for the truth of the conclusion. While the conclusion of a deductive argument is certain, the truth of the conclusion of an inductive argument may be probable, based upon the evidence given.

  • So, what? I read it several times. To me there only are probable things. It may be the case the foundation of all our technology is not correct and there is a minor error. But since it's minor everything is still working now (but it will pose a problem on future technolgoies). – rus9384 Sep 14 at 17:04
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    @rus9384 Given its undeniable successes in every field of human endeavour over centuries past, if you want to convince us that deductive reasoning should be scrapped, you will have to do better than "possible, minor errors." You will have to demonstrate that deductive reasoning inevitably leads to inconsistencies. Good luck with that! – Dan Christensen Sep 14 at 17:33
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    @rus9384 Even a probabilistic analysis in terms of attaching a numerical probability to events could be see an exercise in deductive reasoning. See – Dan Christensen Sep 14 at 17:40
  • @GeoffreyThomas 1. You can also say that P(A) is either equal to 0.1 or it is not, with P(A) being arrived at by using deductive reasoning and the axioms of probability. 2. You made the claim without proof of a possible "minor error" in the foundation of all our technology, i.e. in deductive reasoning. On what do you base this claim? – Dan Christensen Sep 14 at 19:32
  • @Dan Christensen. Thanks for comment. I have thought better of what I said. – Geoffrey Thomas Sep 14 at 20:04

Deductive reasoning is "If this, then that." It is very useful in math and science. It's good for exploring the consequences of beliefs. It's essential in writing software (a conclusion arrived at by inductive reasoning). You're being a bit hypocritical by claiming to disdain it and still using the Internet.

By claiming that arithmetic is taught through induction, you're confusing pedagogy and reality. Arithmetic is a mathematical concept that turns out to be very useful in reality. (There is a mathematical deductive proof technique called "mathematical induction", which has nothing to do with inductive reasoning, which is important in arithmetic.)

You call a triangle a shape with three angles. It's obvious that they have three angles. What's less obvious is that, in Euclidean geometry, the sum of the interior angles is 180 degrees, and I can get into further properties that are even less obvious.

Your complaint seems to be that, given incorrect premises, deduction usually gives incorrect results. No form of reasoning will always give correct results.

  • It is not obvious they have three angles. Makes as much as sense as it is to say "It's obvious that I will wake up tomorrow in 8:30". My complaint is that we can't take anything for sure. And so we should not. However, maybe I just reject the distinction between inductive and deductive, not deductive itself. – rus9384 Sep 14 at 17:52
  • If you define something as having three angles, and it exists in some sense, then it has three angles. I'm completely failing to understand why this might not be so, It's true that, if I try to draw a triangle on paper, it might not have precisely three angles (I don't draw very well), but in that case I haven't really drawn a triangle. Please tell me how it's possible for a triangle not to have three and only three angles, and what relation it has to the fact that I sleep in on Saturdays. – David Thornley Sep 14 at 22:24
  • Relation is single: you just do not gonna call something that has not three angles a triangle. But this statement is not descriptive. It is proclamative. In this sense I reject to say that deduction has anything to do with reasoning at all. Indeed, we can say that if it is raining(1), you are gonna go outside(2), you do not want to get wet(3) and umbrellas allow you not to get wet when it is raining(4), then you should take an umbrella. However, this conclusion is hardly deductive, because no implications exist here, except the conclusion. And this is the only type of reasoning. – rus9384 Sep 14 at 22:41

You summarise what you call deductive reasoning as follows:

Deductive reasoning follows from absolute awareness in own intentions and desires.

This isn’t what philosophers normally call deductive reasoning. The standard version is more accurately portrayed by your opening joke: start with agreed premises and apply accepted logical argument to arrive at conclusions. There is not normally a requirement that deductive reasoners have “absolute awareness in their own intentions and desires”.

As such, and to answer the question in the title: you can safely reject your version of deductive reasoning without affecting your natural discourse with philosophers.

Logical rules and reason are based on causality: if [cause], then [consequence]. For example, a theory (cause) can explain some observable phenomenon (consequence).

Deductive reasoning allows finding consequences having causes (the process is out of the scope of this answer). That is, for example, by knowing a theory, you can find its implications. Another example, by knowing relativity, we know that light will bend in the proximity of a planet. Using deductive reasoning, you conclude that you will have pain if you bash your head against the wall. We can predict relativistic behavior using Einstein's theory.

Inductive reasoning allows finding causes having consequences (the process is out of the scope of this answer). That is, for example, by knowing some general behavior, you can find a theory describing it. For example, if it smells like burning plastic at home, you can conclude there's a problem on the electrical network. If your nose scratches a lot, you can theorize there's polen in the air and your allergy is warning you. Quantum physics was developed by induction: classical physics were not able to predict several observable phenomena, so a theory was created based on observations.

Saying that deductive reasoning should never be used is saying we should never learn. Just theorize the reasons of everything. You are suggesting to destroy the educative system, you are saying that books are useless. That's the reason why gods were created: in order to explain phenomena. That's how religious people think. Not philosophers, not scientists.

  • I am wondering if following is inductive then. 1. There is an inhabited planet in the Solar System. 2. The Solar System is a part of the universe. 3. Therefore there is an inhabited planet in the universe. In either way I guess IRL nothing is completely inductive or deductive. – rus9384 Sep 15 at 9:10
  • Correct. Causality is a mental mechanism, it's not physical; it's our mind which creates links between objects. For example, special relativity can be a result of deduction (it is the result of previous theories and knowledge), or induction (it is also the generalization of facts found with mental experiments). If special relativity would be just 1 (one) idea, perhaps we would be able to detect if it was a cause or a consequence, so if it was induction or deduction. But it's not. Is the result of a huge set of causal reasoning processes, some inductive, some deductive. – RodolfoAP Sep 16 at 8:45

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