Rene Descartes gets a bad rap these days. He is routinely castigated for his naive faith in the power of his method (the basis of the modern scientific method) to penetrate the opacity of nature and yield its underlying patterns and laws. In an age where we have discovered many real limits to pure knowledge - from the quantum uncertainty principle to the persistence of unconscious bias - it is easy to scoff at his reductionism and belief in absolute truth. However, it's a hell of a lot easier for us, 500 years later, to slam him for his enthusiasm than it was for him to turn two thousand years of academic in-breeding on its ear.
Yes, he believed that subjectivity could be overcome through rigorous application of his methods (the "Method of Rightly Conducting The Reason And Seeking Truth in the Field of Science", to be precise), and yes, he believed that through the scientific method we would eventually know all things knowable. In his defence (and it's a hell of a defence), he came up with the revolutionary idea that if you want to understand how something works, it helps to look at it.
What seems to us after three centuries of rationalism as so obvious it hardly bears mentioning was not always accepted as a given. The Greek philosophers, as well as their descendents the Scholastics, were much too sophisticated to actually go out and soil their hands on reality (much like modern economists). Pure logic and a predominant focus on the next world rather than the present one kept the Schoolmen busy calculating how many angels could dance on the head of a pin. Prior to Newton, for example, it was widely believed that the natural state of matter was at rest. This seemed to make sense, because things didn't move unless you pushed them. It wasn't until this theory was tested under a variety of conditions according to Descartes' method that the first law of motion (the natural state of matter is to travel forever unchanged at a constant speed in a constant direction unless acted upon by an unbalanced external force) was discovered.
Interestingly, Descartes' desire not to upset the Church may have influenced many of his own ideas, particularly those of the meditation published after his Discourse on Method had made him famous. Among other things, he invented an implausible theory of vortices to explain how the official shape of the solar system (with Earth at the centre) could be so much at odds with the observed shape of the solar system. If nothing else, this exemplifies the human constraints on objectivity, constraints which Descartes hoped to transcend through the use of his method. That is, he failed to properly apply his own method to many of his inquiries!
One last point of interest concerning Descartes is to drive home the point that he helped to invent science. We don't think of science as a technology, and yet it clearly is, for what is science but a mechanism for abstracting reality by testing theories on how the universe, or a part of it, works?