It is a play on words from the original quote, Don't put the cart before the horse - which basically means do things in their proper order - if you put the cart in front of the horse, you won't get anywhere.
It refers to Descartes sometimes circular logic to explain things (like God and the evil genius). The arguments that establish the trustworthiness of reason (the Cogito Argument) themselves seem to depend on the trustworthiness of reason. We cannot doubt perceptions unless we think of them; but we cannot think of them without at the same time believing they are true, as was supposed. Hence we cannot doubt them without at the same time believing they are true; that is, we can never doubt them.
Its also a joke which is just a play on words. Descartes founded analytic geometry .. and you can't put it before a horse because the horse isn't having any of that. Here it departs from the original meaning of the phrase.
"There was this magnificent mathematical horse. You could teach it arithmetic, which it learned with no difficulty, algebra was a breeze, it could even prove theorems in euclidean geometry, but when you tried to teach it analytic geometry, it would rear back on its hind legs, kick ferociously neigh loudly and make violent head motions in resistance.
The moral of this story is that you can't put Descartes before the horse."