Philosophers often describe one thing as consisting in some other things. However, I've never seen a rigorous definition of the 'consists in' relation. Does such a definition exist?
Apocrypha has it that Quine used to demand he be given the collected whole comprised of the parts of a chicken (i.e. what a chicken consists in). Given the normal thing, a slaughtered and partitioned chicken, he would complain that it was not a collected whole. Given a live chicken he would complain that these were not the parts of a chicken, this was a chicken. (Presumably given a dead chicken, he would claim this was not even a chicken, it was the corpse of a chicken.)
It is impossible to produce what the chicken consists in, without including all the things beyond that that the chicken consist of. If you have any real object, and it is not a chicken, you do not really have everything that a chicken consists in. But a real chicken contains the incidental aspects of the chicken, in addition to the aspects that really make it a chicken, so a real chicken is not what a chicken consists in either.
Various logicians, including Quine himself have tried to make sense of this idea, going back beyond Parmenides and Socrates (in Plato's dialog named after the former) discussing whether a hand is one thing or many things. Is it really made up of the fingers or are they just incidental aspects of it?
To me the entire issue is an overestimation of our power to define things with a single notion. We assume that there is some (extraordinarily complex) statement that would perfectly capture chickenhood or hand-ness. But we are wrong, the definition of a chicken, or a hand, is a convention arrived at by iteration and approximation.
The notion of something consisting in its principal aspects is itself a weak approximation to how meaning works. The statement itself never means anything without example, refinement or reframing. So there is no room here for a rigorous definition.
I think that "consist" expresses actually just the identity relation in disguise. For example, saying "being human consists in being a rational animal" is equivalent to the equality "being human = being a rational animal" or "humanity = rationality + animality".
The "consist" word is used in the context of locating the relevant property among other properties. For example, one considers many particular human beings. Some are short, some tall, some bald, some hairy, etc. And one wonders, where is the humanity in all these humans? And then one is told: "being human consists in being a rational animal". Then one knows better where to look, what to focus on, how to isolate the humanity within the multiplicity of human beings and their many properties.