And inasmuch as the essence of mind consists in the fact (II. xi., xiii.), that it affirms the actual existence of its own body, and inasmuch as we understand by perfection the very essence of a thing, it follows that the mind passes to greater or less perfection, when it happens to affirm concerning its own body, or any part thereof, something involving more or less reality than before.
When, therefore, I said above that the power of the mind is increased or diminished, I merely meant that the mind had formed of its own body, or of some part thereof, an idea involving more or less of reality, than it had already affirmed concerning its own body. For the excellence of ideas, and the actual power of thinking are measured by the excellence of the object. Lastly, I have added "by the presence of which the mind is determined to think of one thing rather than another," so that, besides the nature of pleasure and pain, which the first part of the definition explains, I might also express the nature of desire.
END OF PART III
End of Project Gutenberg's The Ethics [Part III], by Benedict de Spinoza