Martial Gueroult. Descartes' Philosophy Interpreted According to the Order of Reasons, 2 vols., translated by Roger Ariew. University of Minnesota Press. 1984.

Excerpt:

There is, in Descartes’s writings, a seminal idea that inspires his whole enterprise, which is expressed as early as 1628 in the Rules for the Direction of the Mind: it is that knowledge has impassable limits, founded on the limits of our intelligence, but that, within those limits, certainty is complete.  From this a twofold requirement follows: part philosophical–one must seek to determine the limits of our intelligence; part methodological–it is necessary beforehand to doubt everything, but not to doubt our intelligence.   Hence it is the examination of intelligence that will permit us to discover how far the mind can reach: “If someone undertakes to examine all the truths human reason can attain, which, it seems to me, ought to be done at some time in his life by everyone who seriously strives to arrive at wisdom (bona mens), he will discover…that nothing can be known before intelligence, for it is from intelligence that things can be known, and not conversely.”  The statement of this principle, which is formulated in Meditation II in the following manner–the Cogito is the first of known truths, the mind is easier to know than the body, for the mind knows itself without the body, but the body cannot know itself without the mind–ushers in the era of modern idealism and reversed the Scholastic point of view.

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