My sentence is from a philosophical book and it made me perplexed. Such that I wasn't even able to separate the parts of it. So, the sentence is:
Modern philosophy begins with Descartes, whose fundamental certainty is the existence of himself and his thoughts, from which the external world is to be inferred.
By the way, I am also confused about "preposition + relative clause" structure which is used by the author in this book. If anyone could explain this to me I'd be grateful. Thanks in advance.