In his arrival at what he called “the first principle of the Philosophy for which I was seeking,” Descartes famous “I think, therefore I am,” demonstrates the validity of the idea of design. That is, where we find design, we assume thought, and where there is thought, there is an existent being. Here is the excerpt from Descartes, Discourse on Method, Part IV, the first paragraph and part of the second:
I do not know that I ought to tell you of the ﬁrst meditations there made by me, for they are so metaphysical and so unusual that they may perhaps not be acceptable to everyone. And yet at the same time, in order that one may judge whether the foundations which I have laid are sufficiently secure, I ﬁnd myself constrained in some measure to refer to them. For a long time I had remarked that it is sometimes requisite in common life to follow opinions which one knows to be most uncertain, exactly as though they were indisputable, as has been said above. But because in this case I wished to give myself entirely to the search after Truth, I thought that it was necessary for me to take an apparently opposite course, and to reject as absolutely false everything as to which I could imagine the least ground of doubt, in order to see if afterwards there remained anything in my belief that was entirely certain. Thus, because our senses sometimes deceive us, I wished to suppose that nothing is just as they cause us to imagine it to be; and because there are men who deceive themselves in their reasoning and fall into paralogisms, even concerning the simplest matters of geometry, and judging that I was as subject to error as was any other, I rejected as false all the reasons formerly accepted by me as demonstrations. And since all the same thoughts and conceptions which we have while awake may also come to us in sleep, without any of them being at that time true, I resolved to assume that everything that ever entered into my mind was no more true than the illusions of my dreams. But immediately afterwards I noticed that whilst I thus wished to think all things false, it was absolutely essential that the ‘I’ who thought this should be somewhat, and remarking that this truth ‘I think, therefore I am’ was so certain and so assured that all the most extravagant suppositions brought forward by the sceptics were incapable of shaking it, I came to the conclusion that I could receive it without scruple as the ﬁrst principle of the Philosophy for which I was seeking.
And then, examining attentively that which I was, I saw that I could conceive that I had no body, and that there was no world nor place where I might be ; but yet that I could not for all that conceive that I was not. On the contrary, I saw from the very fact that I thought of doubting the truth of other things, it very evidently and certainly followed that I was;
 a piece of illogical or fallacious reasoning, especially one that appears superficially logical or that the reasoner believes to be logical (https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=paralogisms)
 Haldane and Ross (trans.), The Philosophical Works of Descartes, (London: Cambridge University Press, 1977), 100-101.