Because that would be putting Descarte before the Horace.
Explanation: This joke plays on the names of two philosophers and poets: René Descartes, a prominent figure in French philosophy, and Horace, a famous Roman poet. The pun comes from the phrase “putting the cart before the horse,” which means to do things in the wrong order. By substituting “cart” with “Descartes” and “horse” with “Horace,” the joke humorously suggests that studying Descartes before Horace is a mistake, echoing the common expression.