Descartes (1596-1650) employed skepticism as a method of achieving certainty: I will doubt everything that can possibly be doubted and if anything is left, then it will be absolutely certain.
Then I will consider what it is about this certainty (if there is one) that places it beyond doubt and that will provide me with a criterion of truth and knowledge.
His doubting methodology used two conjectures:
1. the dream conjecture - "For all I know, I might now be dreaming."
I pinch myself but am I dreaming that I pinched myself? Might not any evidence I have that I am now awake just be dream evidence? Can I really be certain that the things I see around me - this desk, these arms and legs - have any existence outside my mind?
2. the evil demon conjecture - For all I know, some evil demon had devoted himself to deceiving me at every turn so that I regard its true/certain propositions that are in fact false.
But, even if I am dreaming, I can not doubt that 2+3=5 or that a square has four sides. It seems absolutely certain to me that 2+3=5 and that a square has four sides. But some propositions that have seemed absolutely certain to me have turned out to be false.
So how can I be certain that these propositions (2+3=5; square, 4 sides) or any other proposition that seems certain to me are not likewise false?
For all I know, a deceitful and all-powerful intelligence has so programmed me that I find myself regarding as absolutely certainties propositions that in fact are not true at all.
Thus, Descartes thought that these two conjectures combined to force him to realize that there "... is nothing at all that I formerly believed to be true of which it is impossible to doubt."
As the result of these two conjectures, he believed that he could doubt absolutely everything - except one truth: "I think, therefore I am." (cogito, ergo sum).
Any attempt to doubt one's existence as a thinking being is impossible because to doubt is to think and to exist. The self that doubts its own existence must surely exist in order to be able to doubt in the first place.
Descartes had discovered in the certainty of his own existence an essential characteristic of truth: anything that is as clear and distinct as his own existence would have to be certain.
Using this "clear and distinct" criterion, Descartes was able to certify as beyond doubt his belief in God who would not deceive him. And because it was certain that God would not deceive him, Descartes could then also certify as certain his belief that God would not permit him to be deceived in his belief in the existence of the world outside his mind.
So, then, for Descartes, reality consists of three basic substances: God, Mind, and Matter.
The essential property of a mind is that it thinks; the essential property of body is that it is "extended." Each thought is a modification of Mind; each physical object, a modification of matter.
Since Mind is different from body (otherwise, they would not be two distinct substances), its essential characteristics must be different from those of body. This means that minds cannot take up space or be extended. If they were, they would be forms of body.
Body, in contrast to Mind, is that which is extended. Every form in which a material object can exist can be defined or described in forms of its extensional features - size, shape, position, movement.
Thus, the physical world is conceived of a vast, extended machine. Each past can be described in terms of its geometrical properties.
God is the only substance because God is the only entity which is absolutely independent of all others. "All other things can exist only by the help of the concourse of God."
God is the only creative substance. His Will is the cause of everything that happens in the world or physical realm. God's creative activity accounts for the mind and the body.
Thus, the parts of the physical universe move according to laws that God enacts. Since the universe is basically only extended parts, its activities must be due to an active force outside the material world.
Not only did God create matter, He also conserves and controls it in all moments of history. Modern science can discover and describe this universe.
Descartes' universe is theocentric --> God is the only agent and is the constant source of whatever exists and of whatever happens.
Reality is God and what God wills.
How can the material world interact with the human mind since it is not mechanical?
There is evidence that the two - mind and body - do interact, i.e. a knife cuts and we feel pain.
But how is it possible that the unextended mind and the extended body influence each other?