There is no subject (species or genus, i.e. [a term signifying] a repeated ontologically independent being) which is always connected with 'white' ('is white') or with 'laughs' ('is lauging'), and therefore 'white' and 'laughs' are not propria. This is because we must hold that a given proprium is always on the subject (always connected with its subject, 'attached' to its subject), that is, necessarily so connected. And that will only be manifested when there are more than one such subjects present, and when the proprium is connected to each and everyone of them. The subject thus should be repeated or be repeatable, and thus not particular such as Socrates (interpreted as an individual of the species human being).
We can, it is true, construct (or point to) a set of all things that are necessarily white, but the contrast with respect to 'capable of laughing' lies in the fact that there are also (that is, that in addition to things-necessarily-white) things which are sometimes white, sometimes not. This is not the case for 'capable of laughing'.
A problem could present itself when, for example, a given human individual, cannot, because of, for instance, illness, laugh (in a physiological sense). In this way 'capable of laughing' is not present in all individuals of the species. This can also be the case with 'rational'. Are such individuals then (qua whatness) not humans? Or is 'capable of laughing' not a quale that can represent (in the sense of conceptually replace) the Essence, that is, is it not a proprium after all? And is 'rational' then not an essential quale (differentia) after all?
We can give an answer along the following lines :
Illness of a person corresponds to a perturbation of the dynamical system, leading to phenotypical deviations (that is, to a slightly different course of the system). Normally, however, the system, thanks to its stability, reverts to its original course (and we can laugh again!).