The Enlightenment
The Philosopher Rene Descartes believed that the only way we could save philosophy was to find a new starting point in our thinking. Descartes wanted to find certainty in his thinking. He’s new foundation for thinking rested in systematic doubt.
Only by systematically doubting everything could he be certain he had arrived at certainty. But the one thing that Descartes found out was that he could not doubt that he was thinking, because to doubt that you are thinking means you are thinking that you are doubting. So Descartes became famous for his statement "I think therefore I am" which meant that his logical reasoning could give him certainty. Reason had become the ultimate standard in finding truth. The problem with this view is that if you reject divine revelation then your idea or knowledge of God is nothing more than your own logical construction of who God must be. Self-authenticating, rational self-sufficiency was the basis of Cartesian foundationalism. Truth is then based on our first principles and conclusions. This is also a danger we have today in talking about God when we separate Philosophy from Theology. Without divine revelation our philosophy is just our own assumption of who God must be. If Reason is the ultimate foundation for truth then logic itself would be reduced to an eternal changeless principle of identity. All facts would be wholly known by abstract thinking. If this was true what relationship do our thoughts have with objects in the world and how do they correspond rationally?

The Philosopher Spinoza basically said, "facts simple had to be what the intellect of man using the laws of logic and especially the law of contradiction said they must be.11
. But why must this be? Because if it was not our thoughts would be irrational.
To counteract this supremacy of reason, the empiricist philosophers John Locke, George Berkley and David Hume made the claim that true knowledge came into our minds, we do not invent it. For them there are no innate ideas in the mind. The mind starts of as a blank tablet. Therefore ideas are not a priori but a posteriori, they enter the mind only through experience and reflection. For the empiricists the facts had characteristics in themselves prior to us knowing them, objectivity they thought was guaranteed because the mind receives the facts just as they are. The problem with this view then is that the mind has no ordering of the facts at all. So who is relating one fact to another in a rational system. Ideas and concepts must be related to produce knowledge and if the mind is blank it will be impossible for it to recognizes anything. Also we have the same problem as Aristotle had that there are brute facts out in the world in an impersonal universe with no objective interpretation to them. The truth is our minds have concepts and categories, which we shape all the data that comes in. Our minds are limited by our sensory perceptions. We order and construct data according to our presuppositions and core beliefs. We don’t have a pure access to the facts just as they are because we are doing the interrupting of our observations.

The Philosopher Immanuel Kant believed that objects conform to the operation of the mind and not the mind to objects. For Kant the mind of reason had ultimate autonomy and absolute authority.

The Philosopher Bertrand Russell basically killed these views and all rationality when he said that the source and guide for reason was the accidental collocation of atoms firing by chance.
Friedrich Nietzsche believed that reason was a tool for desire, a tool that can only clarify the stark choices facing all humans. Reason cannot be objective and that in the end all morality and truth are arbitrary. Since every argument must begin with premises that cannot be supported. The original foundation of every argument is merely suspended in air. First principles are simply and only arbitrary choices, if they are indeed the principles upon which all other principles stand. In this regard, first principles cannot be reasonable or rational, if they are genuine first principles. For there would be no principles prior to first principles by which to judge whether the first principles satisfied them as reasonable…It is the intuition that man creates the world, rather than discovers a world in which meaning is fixed. Humans do not find truth but make truth.
For Friedrich Nietzsche reasons were based on human choices and the world was explained by our own subjective interpretations as the world outside us had none. This again does not lead us to a system of certainty in our knowledge.