Essences. Tricky one. In a way this is related to universals, in another way to perspective. Think of a person. Some say the 'essence' of that person is information. That is, they can be wholly described as a pattern of information; and theoretically, be reinstantiated from that information. That information is usually a neurological one (but it could include much more) - a physicist may say that everything (or any particular thing) is wholly described by physics. That is, it can be reduced entirely to a description based on forces, particles, motion etc - it can be described by the categories, terms and concepts of physics.
But there is something wrong here. While we may concede that a planetary system, or a boat race, or an apple, can be described in solely physical terms, we find that this description is only satisfactory from a physics point of view: if one takes a sociological point of view, for example, no matter how much spatial and temporal detail we add into the description, we do not satisfy the criterion or the categories of the sociologist. Some physicists may argue that physics is "all-inclusive" and in fact the sociological factors are just an apparition generated from the physical mechanisms. However, this is to miss the point: the simple fact that physics does not have categories such as value, meaning, fun, nice, makes it unsuitable for certain modes of description. Ergo, it does not contain the whole picture: it is an interpretation. All the other modes of interpretation (eg religion; biology; art; medicine; psychology; choose any field you want) will suit certain situations and give answers which are valid in some specific way. If we begin with physics, pretty soon we must enter the field of biology and psychology if we are wanting to discuss social meaning; and then we will probably need sociology and religion to add the other dimensions to the picture, to make it fully rounded. A mental experience may appear reducible to atoms and forces, but this is only to look at it one way. The atoms are not its essence - they are just one valid description.
The problem is that physics descriptions always stay at the level of physics: but there is so much more to life than this. A human life is not reducible to particles, and not reducible to information. It calls on all those fields, all those levels of understanding. The essence of any "thing" is not in the thing, but dependent on the system we are viewing it from within (yet, it will never be sufficiently described from within any single system; or, any finite combination of systems). Because, as soon as we are abstracting and losing apparently 'extraneous' information, which in fact is only extraneous frmo the point of view we are looking at it.
More importantly, human experience, life from the inside, does not consist of atoms: experience happens at the meta-level, based on macroscopic objects and social processes. These are the constituents of life.