Dugald Murdoch

One of the most influential physicists of the twentieth century, Niels Bohr was born in Copenhagen on 7 October 1885, and died there on 18 November 1962. He came of a well‐to‐do, cultivated family, his father being Professor of Physiology at Copenhagen University, where Bohr himself received his education in physics. After taking his doctorate in 1911, Bohr went to Cambridge University to continue his research on the theory of electrons under Sir J. J. Thomson. After several months in Cambridge, he moved to Manchester to work under Ernest Rutherford, the world leader in the newly emerging field of atomic physics. It was in Rutherford's department that Bohr proposed his revolutionary quantum theory of the structure of the hydrogen atom in 1913.

At the beginning of Meditation Three, Descartes puts forward the proposition that whatever he clearly and distinctly perceives is true. He observes, however, that so long as he does not know whether there is a deceiving God, he has reason to doubt the proposition. Later in Meditation Three, he purports to prove that there is no deceiving God. The difficulty, as Arnauld pointed out, is to see how Descartes avoids reasoning in a circle or begging the question here, for if he can be certain that there is no deceiving God only because he clearly and distinctly perceives this, then he must already be certain that what he clearly and distinctly perceives is true.

This paper suggests that the appearance of circularity in descartes' arguments is due to a lack of precision in his statements of them, Rather than to any flaw in his reasoning. The clear and distinct perceptions presupposed in the demonstrations of the existence of God are not the same as those whose reliability depends upon the existence of god. He is presupposing the reliability only of those clear and distinct perceptions which are known through the light of nature and have metaphysical certainty in that they cannot be imagined false even on the demon hypothesis. And the only clear and distinct perceptions whose reliability he is demonstrating are those which are not known through the light of nature with metaphysical certainty but are subject to metaphysical doubt on the demon hypothesis, Even though they have moral or psychological certainty