I know not what I am made of - you are made of and constituted by atoms

 The real essence of a material thing is its atomic constitution. This atomic constitution is the causal basis of all the observable properties of the thing, from which we create nominal essences. Were the real essence known, all the observable properties could be deduced from it. Locke claims that the real essences of material things are quite unknown to us. Locke’s concept of substance in general is also a ‘something I know not what’.