As with most speculation about William Shakespeare, the idea that his writing springs from a specific philosophy cannot be determined for certain. However, there is a body of evidence that Shakespeare used humanist ideas to inform his writing; humanist sources note that many of the characters speak disparagingly of religion and hold up Man over Gods in a manner inconsistent with the religious, spiritual, and superstitious beliefs of his era. One of the most important pieces of evidence for this stance is also one of Shakespeare's most famous lines:
This above all, – to thine own self be true;
And it must follow, as the night the day,
Thou canst not then be false to any man.
(Shakespeare, Hamlet, gutenberg.org)
This is line is explicitly pro-Man "above all," and can be interpreted as a warning against the habits of religion; if Man is loyal to God above all else, then it is hard to say what is inherently moral and what is simply blind faith. Instead, Polonius cautions to be true to the self, to one's own moral and ethical compass, and make all decisions based on personal principles of integrity. This is not an anti-religious statement, but it is a very pro-secular one, and similar lines in other plays hint that Shakespeare may have simply been a humanitarian, not necessarily a Humanist as defined by modern philosophy.