 One of Locke’s epistemological projects is to distinguish metaphysics and natural philosophy from ethics and mathematics, and one way he does that is to distinguish their subject matters. Metaphysics and natural philosophy are concerned with substances, whereas mathematics and ethics are concerned with modes. For Locke, since an essence accounts for the existence and nature of any being, i.e., “Essence may be taken for the very being of anything, whereby it is what it is”, it follows that the concepts of real and nominal essences apply outside the realm of metaphysics and natural philosophy as well. By his lights, both simple ideas and modes have real and nominal essences. To see this, some definitions are in order.