'mental things, mental factors', are those mental concomitants which are bound up with the simultaneously arising consciousness (citta = viññāna) and conditioned by its presence.
Whereas in the Suttas all phenomena of existence are summed up under the aspect of 5 groups:
the Abhidhamma as a rule treats them under the more philosophical 3 aspects:
Thus, of these 3 aspects, the mental factors (cetasika) comprise feeling, perception and the 50 mental formations, altogether 52 mental concomitants.
Of these, 25 are lofty qualities (either karmically wholesome or neutral), 14 karmically unwholesome, while 13 are as such karmically neutral, their karmical quality depending on whether they are associated with wholesome, unwholesome or neutral consciousness. For details s. Tab. II, Tab. III. Cf. prec. (App.)