It is important to understand that, during Descartes' time, skepticism significantly influenced philosophical thought. Philosophical skepticism is based upon an attitude of doubt toward certain beliefs and that knowledge is limited. This attitude and methodology for thinking led many skeptics to famously doubt the existence of God and even if our own human existence was real or not.
Descartes decided to push on skepticism and "reject as if absolutely false everything in which I could imagine the least doubt, in order to see if I was left believing anything that was entirely indubitable." In other words, Descartes labeled all that was doubtful as false until nothing remained but that which was not doubtful -- the truth. This logical process challenged the skeptics and would become the bedrock of philosophical and scientific methodology henceforth.
Descartes concluded that if one doubts anything at all, the doubting in itself proves that one is thinking -- that one exists:
His dualism made an important separation between physical and metaphysical reality. An important result of this separation was that it allowed philosophers and scientists to study the natural world without having to worry about supernatural questions. Before Descartes, one could not in essence be a Scientist and believe in God at the same time.
Since Descartes' time, many philosophers have argued that we should stop asking questions about God and anything else that we can't verify through observation -- and Descartes would become widely recognized as the father of modern philosophy.
Because of his means of thinking and knowing, Descartes is considered a rationalist -- one who considers knowledge as metaphysical -- existing separate from physical reality -- and that truth cannot be acquired through the senses but through the intellect and deductive reasoning.
Rene Descartes applied his reasoning skills beyond philosophy and science. He was also a great mathematician who, among many other things, developed Cartesian Geometry, the first law of reflection and the use of subscripts to indicate powers or exponents, such as the number 2 for squaring, as in x2.
Descartes' profound impact on the world during and after his time is unquestionable; however, to truly know and understand a philosopher is to know and study their thoughts:
...the nature of man as a combination of mind and body is such that it is bound to mislead him from time to time.
...it is the intellect alone which corrects the error of the senses; and it is not possible to produce any case in which error results from our trusting the operation of the mind more than the senses.
From all this I am beginning to have a rather better understanding of what I am. But it still appears -- and I cannot stop thinking this -- that the corporeal things of which images are formed in my thought, and which the senses investigate, are known with much more distinctness than this puzzling 'I' which cannot be pictured in the imagination. And yet it is surely surprising that I should have a more distinct grasp of things which I realize are doubtful, unknown and foreign to me, than I have of that which is true and known -- of my own self.
I think it will be a good plan to turn my will in completely the opposite direction and deceive myself, by pretending for a time that these former opinions are utterly false and imaginary. I shall do this until the weight of preconceived opinion is counter-balanced and the distorting influence of habit no longer prevents my judgment from perceiving things correctly.
And yet it is surely surprising that I should have a more distinct grasp of things which I realize are doubtful, unknown and foreign to me, than I have of that which is true and known -- my own self.
It does not matter that I do not grasp the infinite, or that there are countless additional attributes of God which I cannot in any way grasp, and perhaps cannot even reach in my thought; for it is in the nature of the infinite not to be grasped by a finite being like myself.
Altogether then, it must be concluded that the mere fact that I exist and have within me an idea of a most perfect being, that is, God, provides a very clear proof that God indeed exists.
Descartes Image by Shannon
Meditations on First Philosophy, by Rene Descartes