Philosophers of modern times have asserted that we know nothing of the essence of bodies. It is affirmed that all that can be known of mind or matter, are merely its properties. Dr. Abercrombie, says, "We talk, indeed, about matter, and we talk about mind; we speculate concerning materiality and immateriality, until we argue ourselves into a kind of belief that we really understand something of the subject. The truth is, we understand nothing. Matter and mind are known to us by certain properties: but in regard to both it is entirely out of the reach of our faculties to advance a single step beyond the facts which are before us. Whether in their substratum or ultimate essence, they are the same, or whether they are different we know not, and never can know in our present state of being." (Abercrombie on the Intellectual Powers. Part I. Sec. I.)

There are many truths which we ascertain by reflection, independently in a great measure of our senses. We are assured and know in our own minds that duration must be endless, and that space must be boundless, not because we have learned these truths directly through the medium of our senses, or have been able to demonstrate them by any process of reasoning. In the same way we know concerning the essence of bodies. Instead of being entirely ignorant on the subject, as modern philosophers assert, it is directly the opposite; we know the essence of all substances. Solidity is the only essence in existence. Although the ultimate atoms of matter cannot come under the cognizance of our senses, and we cannot demonstrate their solidity by any process of reasoning, yet we are none the less assured of their solidity. We believe that they are solid because it is impossible for us to believe otherwise. We are as certain that the ultimate atoms of all substances are solid, as we are that they exist. What we mean by solidity is, that all substances completely fill a certain amount of space, and that it is impossible for them ever to fill a greater or less amount of space.

The amount of absolute space occupied by any substance is constant, that is the elementary atoms cannot be increased or decreased in magnitude in the least degree. Particles may be divided, but their respective parts occupy the same amount of space when separated as when united. Condensation or expansion is not a property of the ultimate atoms of bodies, but merely the relation which these atoms sustain to each other. When a collection of atoms called body are forced into a closer connexion with each other, the body is said to be condensed. When their relative distances are increased the body is expanded. The maximum of density excludes all pores. In such a condition the space is wholly occupied—any further condensation is absolutely impossible. A bar of iron varies its dimensions with its temperature, while the atoms of which the bar consists remain unchangeable in size. The pores of the iron increase in the same proportion as the bar increases, and diminish as the bar diminishes. Solidity is universally supposed to be a property of atoms, but this is an error. Solidity is not a property, but only another name for the essence. A property must be a property of something; but solidity is not a property of anything—it is the essence itself—the thing that exists, aside from all properties and powers. If we suppose solidity to be a property, then it is evident that there must be a distinction between atoms as possessors, and solidity as the thing or property possessed; but we find it impossible to conceive of atoms separate and apart from solidity. Deprive atoms of solidity, and they are deprived not of a property, but of existence itself, and nothing remains. Solidity is associated with existence, and we cannot conceive of the one independently of the other. Solidity, then, is the essence to which all qualities belong—taste, smell, colour, weight, &c., are the affections of solids. Every feeling or thought is the feeling or thought of solids. All the powers of the universe, from the almighty powers of Jehovah down to the most feeble powers that operate, are the powers of solid atoms. We can conceive of solid atoms existing without powers, but we cannot conceive of atoms existing without solidity; therefore the very essence of all substance is solidity. Love, joy, and all other affections are only the different states of this essence.

When the essence or solidity of substance is considered by itself, independently of its powers, there cannot possibly be any difference in atoms only in their magnitude and form. The essence of all substance is precisely alike when the essence alone is considered. Substances can only differ in their magnitude, form, and susceptibilities, but not in their essences, for they are and must be alike.