# How can we justify the use of logic?

I would like to hear some philosophical arguments which justify the use of logic.

Additionally, if a justification of logic takes the form of an inference, is it a problem if this inference itself makes use of logic.

Can we justify logic in a way that is not circular? Is a circular justification a bad thing?

Thanks.

• How about limited resources? It's just too wasteful to make decisions based on reading chicken entrails. Commented Mar 3, 2015 at 21:55
• I recall that there is an evolutionary biologist who purports to be able to explain logic on evolutionary grounds; IIRC, the claim is that in some sense, classical logic is the most adaptive method of assigning an element of {True, False} to various statements. Of course, this argument is circular, because it uses logic to explain logic, which kind of prevents it from being a stand-alone explanation of logic. Anyway, you should Google it, and try to find the book that he wrote. Commented Mar 3, 2015 at 22:56
• You can see Michael Dummett; at least The Justification of Deduction (1973) in Truth and Other Enigmas (1978) and The Logical Basis of Metaphysics (1991). Commented Mar 4, 2015 at 9:11

This is the very topic of Hegel's Science of Logic, highlighted neatly in the introduction Allgemeiner Begriff der Logik. To some extent he says that: yes, ultimately it is circular, but no, this does not mean that there is no way to get to the heart of the matter anyway. On the other hand, few are able to follow the path he suggests.

• Can you expand on what Hegel's arguments are for these statements? Commented Mar 5, 2015 at 8:41
• I did add the pointer, just look at the very first paragraphs, notably 34 they are quite clear. ncatlab.org/nlab/show/Science+of+Logic#34 Commented Mar 5, 2015 at 15:09

What kind of justification are you looking for? Logic is the means of correct reasoning. Different logics (formal systems of inference) exist which are basically attempts at logic as I've defined it (or other useful formal systems). The idea is to use means of reliable inference. To accept illogical inference is to accept unreliable inference. The justifications for wanting or using reliable inference are justifications for logic.

Others will I'm sure provide historical discussions concerning justifications for the use of logic. But personally I don't think there's any real reason to do so. If logic isn't applicable then why bother arguing? There is no normative grounds to my statement carrying any force of truth, so connecting sentence A to conclusion B is just a bit of writing with no real content behind it.

Maybe you're happy to accept that conclusion. If so, cool - enjoy the aesthetics of the scribbles!

So far as I understand the question, my answer is "no" (to both clauses). And I do not think that answer is at all surprising either. As is the case with any justification of anything, one could ask for a justification of the justification, and at some point, you must either hit a wall or fall forever into an infinite justification regress. (Why does 2 + 2 = 4? Well, because it follows from the concepts of a unit, addition, and identity. Why should something "follow" like that? Logic. Why should I accept logic? Well, it just makes sense, doesn't it?) Personally, I don't know of anyone who has opted for the infinite fall. If you want to function as a thinking human being, you simply must accept at some point that there are some things you will never be able to prove; the consensus seems to be that logic in the general sense (or, at the least, some set of "first principles" that will include propositions about logic) is that "ground level" - that thing that just has to be accepted.

Now, I might be able to give you a better answer about certain formulations of logic: every logician (as far as I know) who defends or proposes a rule of logic undertakes at some point to explain why his formulation is true - see Aristotle's logic, for example. Such explanations often appeal to intuition, which is more or less just a sophisticated bet that no reader, once he or she understands the author, will be able to imagine an objection or counterexample.

I would like to hear some Philosophical arguments which justify the use of logic.

Additionally, if a justification of logic takes the form of an inference, this inference itself makes use of logic.

Can we justify logic in a way that is not circular, is a circular justification a bad thing?

Logic can't be justified. Nor can anything else.

An argument sez that if some set of assumptions are true and certain rules are valid, then the conclusion is true. There is no known way to ensure that those conditions hold, and so there is no known way to prove the conclusion of an argument. In particular, no argument can do the work of justifying argument itself since the assumptions and rules would have to be shown to be true for the conclusion of that argument to be proven.

The only known solution to this problem is to reject the demand for justification root and branch. All of our knowledge is just guesses controlled by criticism. For more on why justification is piffle and what should replace it, see "Realism and the Aim of Science" by Karl Popper Chapter I.

There is more than one kind of logic. Besides classical logic there are also, for example, paraconsistent logics, modal logics and intuitionistic logics. That means we have choices available to us of which logic to use for any particular class of statements. The question of justifying logic can be rephrased, to emphasize the importance of the task, as justifying which logic to use for a particular class of statements.

Michael Dummett makes a distinction between realist and anti-realist positions regarding particular classes of metaphysical statements, such as mathematical statements, ethical statements, statements about the past or the future to name a few. Furthermore he notes that circularity is not acceptable: (page 8)

The realist thesis is not a possible object of discovery alongside the propositions it proposes to interpret: it is a doctrine concerning the status of those propositions.

This doctrine involves a principle of logic: (page 9)

It is difficult to avoid noticing that a common characteristic of realist doctrines is an insistence on the principle of bivalence - that every proposition, of the kind under dispute, is determinately either true or false. Because, for the realist, statements about physical reality do not owe their truth-value to our observing that they hold, nor mathematical statements their truth-value to our proving or disproving them, but in both cases the statements' truth-value is owed to a reality that exists independently of our knowledge of it, these statements are true or false according as they agree or not with that reality.

Taking a realist or anti-realist position amounts to taking a position on a logical principle with respect to a specific class of statements. Hence justifying the logic we use involves justifying a metaphysical position about that particular class of statements. Since the realist thesis is not justified "alongside the propositions it proposes to interpret" it would require other informal arguments or a logic more neutral than the one being used with the class of statements.

Can we justify logic in a way that is not circular? Is a circular justification a bad thing?

One cannot use the logic used to study a class of metaphysical statements if the logic itself is ultimately being questioned. Hence an informal argument or a more neutral logic would be needed.

Dummett, Michael, The Logical Basis of Metaphysics, 1991, Harvard University Press.

My justification for the use of logic is that logic allows us to eliminate (reduce) "erroneous paths" to truth. Thereby allowing us to get as close as it is possible, to the truth.
My justification is a conclusion, not an inference, therefore it is not circular.

How can we justify the use of logic?

Humans became aware of the existence of logic because of Aristotle's syllogistic.

We use logic not because we decide to use it. We just do it. Most of the time, we don't even know we are doing it. And we do it all the time.

We also use memory. Should we justify the use of memory? Vision? I use my bottom to sit. Should I justify that?

Well, I suppose what you really meant was: How can we justify the use of formal logic?

I hope this will be enough of a clue.

The reason for believing the premises must be in virtual of facts. Psychologically, it is an inductive process. Take "the law of thoughts" for example:

Apart from the special doctrines advocated by Kant, it is very common among philosophers to regard what is a priori as in some sense mental, as concerned rather with the way we must think than with any fact of the outer world. We noted in the preceding chapter the three principles commonly called "laws of thought." The view which led to their being so named is a natural one, but there are strong reasons for thinking that it is erroneous. Let us take as an illustration the law of contradiction. This is commonly stated in the form " Nothing can both be and not be," which is intended to express the fact that nothing can at once have and not have a given quality. Thus, for example, if a tree is a beech it cannot also be not a beech ; if my table is rectan- gular it cannot also be not rectangular, and so on.