In short, he attempts to map out the whole field of human knowledge, and to tell those who work in this corner of it or in that how they should set about their task.
As for Descartes, he writes of the trustworthiness of human knowledge, of the existence of God, of the existence of an external world, of the human soul and its nature, of mathematics, physics, cosmology, physiology, and, in short, of nearly everything discussed by the men of his day.

No man can accuse this extraordinary Frenchman of a lack of appreciation of the special sciences which were growing up.

No one in his time had a better right to be called a scientist in the modern sense of the term.

But it was not enough for him to be a mere mathematician, or even a worker in the physical sciences generally.

He must be all that has been mentioned above.
The conception of philosophy as of a something that embraces all departments of human knowledge has not wholly passed away even in our day.