We are simply contesting his right to posit certain first principles

Among the superstitions from which we are freed by the abandonment of metaphysics is the view that it is the business of the philosopher to construct a deductive system. In rejecting this view we are not, of course, suggesting that the philosopher can dispense with deductive reasoning. We are simply contesting his right to posit certain first principles, and then offer them with their consequences as a complete picture of reality. To discredit this procedure, one has only to show that there can be no first principles of the kind it requires.
As it is the function of these first principles to provide a certain basis for our knowledge, it is clear that they are not to be found among the so-called laws of nature. For we shall see that the 'laws of nature', if they are not mere definitions, are simply hypotheses which may be confuted by experience. And, indeed, it has never been the practice of the system-builders in philosophy to choose inductive generalizations for their premises. Rightly regarding such generalizations as being merely probable, they subordinate them to principles which they believe to be logically certain.
This is illustrated most clearly in the system of Descartes. It is commonly said that Descartes attempted to derive all human knowledge from premises whose truth was intuitively certain : but this interpretation puts an undue stress on the element of psychology in his system. I think he realized well enough that a mere appeal to intuition was insufficient for his purpose, since men are not all equally credulous, and that what he was really trying to do was to base all our knowledge on propositions which it would be self-contradictory to deny. He thought he had found such a proposition in 'cogito', which must not here be understood in its ordinary sense of 'I think', but rather as meaning 'there is a thought now'. In fact he was wrong, because "non cogito' would be self-contradictory only if it negated itself; and this no significant proposition can do. But even if it were true that such a proposition as 'there is a thought now' was logically certain, it still would not serve Descartes's purpose. For if 'cogito' is taken in this sense, his initial principle, 'cogito ergo sum', is false. 'I exist' does not follow from 'there is a thought now'. The fact that a thought occurs at a given moment does not entail that any other thought has occurred at any other moment, still less that there has occurred a series of thoughts sufficient to constitute a single self. As Hume conclusively showed, no one event intrinsically points to any other. We infer the existence of events which we are not actually observing, with the help of general principles. But these principles must be obtained inductively. By mere deduction from what is immediately given we cannot advance a single step beyond. And, consequently, any attempt to base a deductive system on propositions which describe what is immediately given is bound to be a failure.
The only other course open to one who wished to deduce all our knowledge from 'first principles', without indulging in metaphysics, would be to take for his premises a set of a priori truths. But, as we have already mentioned, and shall later show, an a priori truth is a tautology. And from a set of tautologies, taken by themselves, only further tautologies can be validly deduced. But it would be absurd to put forward a system of tautologies as constituting the whole truth about the universe. And thus we may conclude that it is not possible to deduce all our knowledge from 'first principles'; so that those who hold that it is the function of philosophy to carry out such a deduction are denying its claim to be a genuine branch of knowledge.

The function of philosophy [fragment]
uit: Language, truth and logic - A.J. Ayer