However, the two schools reach a parting of ways at this point. They disagree with what should constitute the foundation of knowledge. The rationalists argue that reason, innate concepts and deductive reasoning should be the structure of knowledge while the empiricists argue that sense data, perception and inductive reasoning should become the foundation of knowledge.
The great rationalist, Rene Descartes in his work Meditation had found a foundational belief of which we cannot doubt – Cogito ergo sum (‘I think, therefore I am’). He argued that while we can doubt everything in the world, there is one thing which we cannot doubt, and that is the belief that there is an entity, I, which is doing all the doubting. For the mental process of doubting to exist, there must be one entity which is doing it. This, he claimed, will be the foundation of knowledge. Some other rationalists also argue that humans are born with innate ideas, mental contents that are presumed to exist in the mind prior to and independently of any experience. As there are contents which cannot be obtained through experience, the rationalists argue that the reliance on reason is superior to reliance on experience, and thus should be made the foundation of knowledge.
The empiricists attack the rationalist by saying that while there are knowledge which can obtained through reason alone, they are of no use to human. They argue that knowledge by definition does not tell us anything useful about the world, for these are defined by humans. The empiricists also deny the existence of innate ideas. Instead, John Locke, the British empiricist, maintained that humans are born with our mind much like a blank slate, or tabula rasa, and that when we experience things, these experiences are being written on this blank slate, building our knowledge. Some empiricists have argued that it is indubitable that we can perceive and we are having experience. For instance, while it can be doubted if I am reading a passage, it is indubitable that I am having the perception of a passage written on a piece of paper (or on the computer screen). This, arguably, can be made the foundation of knowledge as they are indubitable.
While both rationalism and empiricism differ in their view of having different foundation of knowledge, it is observed that both do not make any significant headway in completely defeating scepticism. While both found the indubitable 'foundation', the empiricists maintaining that it is indubitable that we are having perception and the rationalists maintaining that we cannot doubt the existence of 'I', both of them have been confined to these foundational beliefs. The rationalist could not find a way to extend the belief of 'I think, therefore I am' to build the structure of knowledge. Descartes had relied on the existence of God to do so. He argues that God being omniscient and benevolent would grant us certain clear and distinct ideas which cannot be doubted and with these clear and distinct ideas, we can gain knowledge through deductive reasoning. However, till now, the existence of God has yet to be satisfactorily proven. So in general, the rationalists have yet to be able to claim with confidence that their way can solve the regress problem and path the way towards gaining indubitable knowledge.
Empiricists face the same problem of extending their so called 'foundational beliefs' to building knowledge for they have to rely on inductive generalisation to do so. Induction and causality have faced serious attack for they are not self justifiable. Though for the all the time known to human, the law of gravity has been working, who are we to say that tomorrow it will still work? The justification we would give is no more than induction itself. Hence, the empiricists have also been not able to defeat scepticism.
Descartes in his First Meditation, outlined an Evil Demon argument. He argued that we might be constantly duped by an all-powerful evil demon so that we will never be sure of everything around us. We can never be sure that the external world, or so called 'reality' really exist, not even that our very own body exist, for this Evil Demon might be feeding false impression and information to your head, making you feel as though you and the world exist. The rationalists and empiricists both fail to really defeat this argument. The rationalist can only be sure that there is entity that is doing all the doubting. Descartes had relied upon the existence of God and the so called 'clear and distinct idea' to establish the truth of reality from the foundational belief of 'I think, therefore I am.' However, since Descartes' argument for the existence of God had been seriously doubted, his argument is therefore considered unconvincing in general. The empiricists, on the other hand, could only establish the fact that we are having perception. They could not ascertain if the reality existed.
In conclusion, the rationalists and empiricists, despite their obvious difference in methodology, they are in fact similar in a lot of aspects. Both of them agree that a foundation belief is the key towards the regress problem, and that both of them have not been able to satisfactorily defeat the sceptics. The main difference lies in the fact that they have different ideas of what should constitute the foundational belief.
