Some people claim that all assumptions should be questioned, and even that the questioning of assumptions is at the heart of philosophy.
Probably, this is an assumption that needs questioning.
First of all, what is an assumption?
My online dictionary says the following (in the sense that is relevant here)
"The act of taking for granted: assumption of a false theory.
Something taken for granted or accepted as true without proof; a supposition: a valid assumption. "
Notice that according to this definition, an assumption might very well be true. It does not have to be false. And, also, although it might be taken as true without proof, that does not mean that the assumption may not be backed with good reasons which may, however, fall short of proof. In fact, I might have very good reasons for making an assumption.
For example, let’s just take a mundane case of that. Suppose I say, “I assume that Joe is going to the party.” If someone asks why I assume that, I need not be speechless. I might say, “Well, because this party is being given by Jane, and Joe likes parties given by Jane, and nearly always goes to them”. Of course, this is a trivial (non-philosophical) case. But when we do philosophy, it is usually a good thing to remind ourselves how a certain concept is usually used before we go esoteric.
So, an assumption may be made, but it may have very good reasons to back it up.
Now, is it true that all assumptions are wide open for questioning? Well, maybe open, but not “wide-open”. What I mean is that if you are going to question an assumption, shouldn’t you have some reason to question it. Descartes thought that everything could (and should) be doubted. And people who claim that all assumption should be questioned are, I think, the heirs of “Cartesian Doubt”. I mean suppose I, out of the blue question not merely whether the Earth has a particular shape (say) square, or round, or trapezoidal, but whether the Earth has any shape at all, because it is an [/i]assumption that the Earth has any shape at all, I might reply, “Well maybe it is an assumption. But it is not something for which I have no good reasons. After all, the Earth is an object in space, and all objects have some shape.” Now it is true that even my reason for thinking that Earth has some shape, namely that it is an object, is “an assumption”. But so what? Although, the person who makes assumptions does have the responsibility to back up his assumptions, doesn’t the questioner also have the responsibility to back up his questions? For instance, what makes the assumption-questioner think that Earth has no particular shape, and is not a physical object in space? If the questioner has not reasons for his doubts, then isn’t he like the child who, after each of his questions beginning with “why?” just asks, “why?” again? Why does the questioner not think that Earth is a physical object in space? Or is it that he is taking for granted, but without any good reason, that he can simply question, and has no responsibility for backing up his questions?