The 'residing in' in this or that, in the sense of : that we can consider the Essence in Socrates, or (if we prefer it) in Plato, or (if we prefer it) in Melissus, or (if we prefer it) in Peter, etc., meaning that it can be encountered in all these individuals, is, according to me, precisely one of the necessary conditions (for something) to be determined at all (i.e. involving a definite content at all). This 'residing in' to be necessary implies that the ' individual ' (that is, the status of ' individual ' , or (the status) of ' to be an individual case ' ) is necessary, necessary for some (given) content to be definite.
' Man ', to take an example, really signifies a definite something, that is to say, a definite content, not until it is repeated (or is repeatable). Only then we have to do with regularity and not contingency. Such a content is thus constant when we go from one individual (as a member of a certain set) to another, and not intrinsically one-off. A given something thus should be, in order to be definite, a case (an individual instance) among several (possible) cases. One-offness (absolute singularity, particularity) is nothing more than a random fluctuation and is totally engaged in ' passing by '. Consequently it then is not something. " Is " here means ' remains ', and demands a certain stability and repeatability ( It could also be the case that we already have definiteness if we are, as regards a unique content that is under discussion in this respect, allowed to speak of a certain 'durée' (duration) in Time, a certain persistence of this content in Time, because this already is a kind of repetition, and if not, there is already some stability present, expressed in this 'durée' ). In fact a repetition in Time as well as a repetition in Space is necessary, because the definiteness of something means that it reacts in this particular way, that is, in a definite way, according to prevailing circumstances, where and when it is encountered. A definite content, that is, something having a definite content or nature, behaves in all individual cases, and at all times, in the same way under the same external circumstances ( This amounts to the presupposition of every material thing having a Species-Individuum structure, as discussed in the document The Species-Individuum Structure in the Critical Series of documents in First Part of Website ).
Accordingly being individual belongs to the Essence [of, to continue our example, man] (this Essence in Plato, or in Socrates, or ... ), while being particular does not belong to the Essence. The being individual belongs to the Essence because it must be 'a case' out of many possible cases. And such a case is individual, it is an individual case. The being individual (as a partial aspect of the Essence) does not play any role in the distinction of whatever given Essence from any other qualitatively different Essence. Individuality is a condition for (something to be an) Essence ( = definite Essence). Individuality (in the case of man, traditionally considered) implies either Socrates, or Plato, etc., thus individuality implies repetition and thus also definiteness. Being particular, on the other hand, is being unique and doesn't imply definiteness ( Something is definite if it always [that is, in all cases] behaves so and so [or looks so and so, or reacts so and so, etc.] ). Something that is particular is not repetitive, and is therefore contingent.