Descartes attempted to doubt everything away that he had, up till then, held to be true, in order to start philosophy off on the completely new ground. In order to do this he imagined that even the solid truths of mathematics might be doubted. The way he did this was to suppose the possible existence of a super-powerful agent that had the absolute, god-like power to delude the human mind into thinking, for example, that 2 + 2 = 4, even though the evident truth about such an equation was that 2 + 2 actually, in the real world, equal 5.
The question is, is such a radical method of doubt, that hypothetically supposes the entire solid science of mathematics to be possibly false, itself based on solid reasoning? Isn't it a lot more reasonable to suppose, even if the human race never existed, that two apples and two oranges make up four pieces of fruit? (Or not?) Use your imagination to answer this question.