It is a matter of tacit consensus that rationalist adeptness in urban planning traces its foundations to the philosophy of the Renaissance thinker and mathematician René Descartes. This study suggests, in turn, that the planned urban environment of the Renaissance may have also led Descartes, and his intellectual peers, to tenets that became the foundations of modern philosophy and science. The geometric street pattern of the late middle ages and the Renaissance, the planned townscapes, street views and the formal garden design, appeared as parables for the perfection of the universe and the supremacy of critical reason. It is within this urban metaphor that Descartes's philosophical narrative betrays perceptual and conceptual impact from the contrast between convoluted medieval townscapes and the emerging harmonious street patterns where defined vistas and predictable clarity of street views were paramount. The geometrically delineated street views of the Renaissance new town became the spark that lit the philosopher's sagacity in reflecting upon the concept of "clear and distinct ideas." Past suggestions that Descartes was led to his philosophical breakthroughs through his discovery of co-ordinate geometry reinforce further the stance that Renaissance planning predisposed rationalist thought.