The central theme of these thinkers, philosophers and scientists has mainly been providing a scientific basis for the need of order in society. A scientific foundation is being constructed in their writings through a concept of self and causal reasons behind existence of a person. This quest, in effect, finally led to the emergence of very strong strands of individualism and basic rights of the individual, namely, right to life, liberty and property.
Main tool of their, especially, Rene Descartes' pursuit of 'self' and 'a person's existence' is reason linking human nature, natural law and natural rights. This human reason as a faculty of human mind formulates a scientific and epistemological paraphernalia for 'self concept' and Godly human 'being'. Locke in his An Essay Concerning Human Understanding and Jacques Rohault in his Ultimate Questions: Thinking About Philosophy use primarily 'reason' as their major tool for scientific research and methodological strength.
Descartes in the first book of his Meditations leads the reader first to skepticism and then attempts to offer a solution. He initially expresses doubt for 'being' and goes further to prove the reality of 'being'. For him, when we doubt a thing or being, we then ipso facto, by virtue of the existence of 'doubt', must know that what we doubt exist as a reality. Doubt we must for Descartes. Otherwise, all apparent reality is a dream or an illusion. This is not an instance of contradiction in Descartes. Here, it is the order and tool of argument is important than any other thing.
Descartes' critics such as Johannes Caterus, Friar Marin Mersenne, Thomas Hobbes, Antoine Arnauld, Pierre Gassendi and Jesuit Pierre Bourdin point out quite a few criticisms. For them:
1. If we think of a perfect being - God or God like - as suggested by Descartes, it does not mean that such a being really exists.
2. How can we be sure that what we think is a clear and distinct perception really is clear and distinct
3. Contrary to what Descartes argues, we are certain that bodies exist/that perception coincides with reality.
4. We don't know that we know everything about the mind.
These criticisms do not appear to hold ground for they must first grasp what Descartes, Jacques Rohault and Locke generally try to present as their method and epistemology. What further they say and go ahead with is not so important as their epistemology providing a scientific materialist foundation for larger discussions.
There are so many relatively technical terms involved in this quest for human and individual's self and existence and identity of a human person.
Theirs' is also a humanistic perspective of self concept focusing on 'I', 'You', 'Your' and our experiences. As such they are highly philosophical and empirical. The philosophy of empiricism necessitates its stress on human experiences. Human emotions and sensations are also experiences though they by themselves do not form the human body and mind.
Locke is a bit away from this Cartesian logic. He uses his tools of 'intuition', 'demonstration' and