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The fact that we know we have a great great grandfather. Is the reasoning we use for this inductive or deductive?

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    Inductive; for all we know we could have been made in a lab and every piece of evidence for our having a great great grandfather is fabricated. It also relies on ideas from science, which, while they are probably true, are gathered via inductive reasoning about the world. Facts such as "things cannot spontaneously generate" and so on. Those are understood inductively.
    – Not_Here
    Commented Apr 13, 2017 at 4:17
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    An argument such as "every person has a great great grandfather" "I am a person" "therefore I have a great great grandfather" is deductive, but the justification for "every person has a great great grandfather" is inductive. I guess to that extend the answer to your question depends on the specifics of the argument you are asking about. Also, see here iep.utm.edu/conf-ind
    – Not_Here
    Commented Apr 13, 2017 at 4:19

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You don't use induction, because induction is impossible:

Deduction vs Induction -- are they equally valid?

Some deduction may be involved in working out the consequences of existing biological knowledge. That knowledge was created by noticing problems with previous ideas, guessing solutions to the problems and then criticising the guesses until only one is left and it has no known criticisms.

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    if induction is "imppssible" you'll have to explain how we routinely use it to great effect.
    – user20153
    Commented Apr 13, 2017 at 20:27
  • You don't use induction. Criticisms of inductivism (the belief in induction) are explained at the answer I linked. No answer has been given to those criticisms. There is an alternative that I pointed out above and in the linked answer, of which there are no unanswered criticisms.
    – alanf
    Commented Apr 13, 2017 at 21:52
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    I think that a single stack exchange answer without unanswered criticisms for a month is a small justification for a claim that a nearly-universally accepted form of reasoning talked about by philosophers for millennia doesn't exist. At best, you could make the claim that scientific theories are not truly induction, which is where Popper went with it.
    – Cort Ammon
    Commented Apr 14, 2017 at 21:01
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    Popper's refutation of inductivism and his proposed alternative has had no unanswered criticisms for about 80 years. Popper's claim was not that scientific theories are not induction. That was just a consequence of his unrefuted argument that induction is impossible. And the fact that philosophers refuse to give up on inductivism is a mystery since they have no answers to the criticisms of inductivism. Indeed, most philosophers can't even state those arguments all the alternatives, as you have illustrated with your inability to state the argument properly.
    – alanf
    Commented Apr 15, 2017 at 11:46
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You did not in fact use any reasoning at all. You stated a fact. The reasoning is involved in the justification of that fact, which was not included. It could be any justification at all. Some popular ones include:

  • Deductive logic: If you start from the claim that every person has a father (and elide any issues of Adam and Eve for a moment), it's easy to argue that every person has a grandfather, and a great grandfather, and a great great grandfather. Taking that last claim, "every person has a great great grandfather" and "I am a person" you can deduce that "I have a great great grandfather."

  • Inductive logic: If you are certain that you have a father, and that they have a father, you may induce that probably every one of your ancestors had a father, and we can build a chain out of them. So long as you're comfortable assuming that chain is long enough, you can recognize that you must have someone in the "great great grandfather" position.

  • Abduction: It's entirely possible that you don't have a great great grandfather at all. You might be a test-tube baby from entirely synthetic sources with no paternal lineage to speak of who was adopted under conditions of absolute secrecy. Of course, that hypothesis seems much less likely than the one that when your father says he had a father before him, and so on and so forth, and your great great grandfather's name was Joe, and lived from 1819-1887 and is burred in Kentucky. So, you may use abduction to assume that the most likely hypothesis is indeed true. Thus, you have a great great great grandfather.

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You don't actually mention the reasoning, so it's difficult to say what type of reasoning you are using. I'm guessing that you mean abductive, which basically means reasoning to the best explanation. It's generally used in science - where we take the evidence we have and formulate the best explanation we can.

In this example, you would presumably look around and see that people are related to one another, that parents produce babies - that sort of thing.

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