And just as in tearing down an old house, one usually saves the wreckage for use in building a new one, similarly, in destroying all those opinions of mine that I judged to be poorly founded, I made various observations and acquired many experiences that have since served me in establishing more certain opinions.
Descartes, Discourse on Method (AT VI, 29), Cress, tr.
This point is often overlooked in criticisms of Descartes. He makes it again in the Replies to the Seventh Objections, in his elaborate parable of the architect: the fact that you tear down a building doesn't mean you can't keep around materials from it to re-use in a better building. And it's a key part of what Descartes is doing; it can't be ignored.
ADDED LATER: Here's the relevant section from the Seventh Replies (AT VII, 539-540); 'he' is the "jobbing bricklayer" in Descartes's story who, jealous of the architect and not entirely bright, is constantly mocking the architect for supposed inconsistencies:
In scene three, he represents the architect as showing him the stone or rock at the bottom of the trench -- the rock on which he intends that his entire building shall rest. But he picks up the rock with a sneer. 'This is excellent, my distinguished friend: you have found your Archimedean point, and without doubt you can now move the world if you so wish. Look: the whole earth is already shaking. But since, I gather, you are cutting everything back to the bone, so that your method may include only what fits and is coherent and necessary, may I ask why you keep this stone? Did you not order us to throw out the stones with the sand? But perhaps it slipped in by accident....' And later on, when the architect sees some rough stones, which had been thrown out of the trench with the sand, and collects them so that he can use them in the building, his opponent makes a joke of this.
Descartes is here loosing all cannons against his opponent, Fr. Pierre Bourdin, SJ, who wrote a mocking series of criticisms of Descartes in a comic dialogue; the jobbing bricklayer echoes Bourdin's criticisms. Descartes was very angered by Bourdin's objections; if you can't tell from his scorn, you can tell from the fact that he wrote a letter of protest to Bourdin's superior, the Provincial of France. That Bourdin was a Jesuit made the matter worse; one of Descartes's lifelong projects was to persuade the Jesuits to teach Cartesian philosophy. Descartes was so angered by the objections that he considered not responding to them, but because Bourdin's criticism might be taken as representing the attitude of the Society of Jesus itself, Descartes felt he had no choice. Bourdin's criticisms are actually not very far from criticisms that are commonly made of Descartes, so it's really a pity that so few people read the Seventh Objections and Replies. Regardless, you'll note an indirect reflection of the importance of recycling of ideas for Cartesian method in the mockery of the passage above.