The training of the mind in a logical, orderly manner has been shown to be a very effective way of defending yourself from psychic damage. As you progress in the skill, your resistance to psychic attacks will increase. You'll also notice that your willpower recovers more quickly as your skill in philosophy rises. Those who Transcendent the teachings of Philosophy will gain the ability to participate in psychic combat, a mini-game played with the mind. Reading HELP PSYCOMBAT will explain the details.