From: The Golden Sayings of Epictetus (translated by Hastings Crossley) - from Project Gutenberg.
The beginning of philosophy is to know the condition of one's own mind. If a man recognises that this is in a weakly state, he will not then want to apply it to questions of the greatest moment. As it is, men who are not fit to swallow even a morsel, buy whole treatises and try to devour them. Accordingly they either vomit them up again, or suffer from indigestion, whence come gripings, fluxions, and fevers. Whereas they should have stopped to consider their capacity.