"indefinable notion" is a correct and usable phrase in written English.
You can use it to refer to an idea or concept that is difficult to describe or explain. For example, "The feeling of joy I get when I'm outdoors is an indefinable notion that I can't quite put into words."
This is not a naturalist definition, since 'suitability' is itself a 'unique and indefinable' ethical notion (FE 146; also 159).
Since 'good', like 'yellow', is not a complex notion, it is indefinable.
His ad hoc approach to monetary policy is indefinable, allowing him to foster the notion he is clairvoyant.
Ackermann set theory is a theory of classes in which some classes are sets, but there is no simple definition of which classes are sets (in fact, the whole power of the theory is that the notion of set is indefinable!) One can conveniently add axioms of Foundation and Choice to this system.
There's just no basis for belief in some indefinable, intangible, have-it-or-not attribute.
Reader Eric thought it would be an impossible job for most people: "I think it is silly to entertain the notion that we can 'train' charisma into someone, as if this indefinable quality could be checked off on your career to-do list," he commented.
The first step in the analysis of the existent (al-mawǧūd)[6] is to acknowledge its absolute priority: the existent as such is indefinable (Ilāhiyyāt, I, 5, 29, 5 31,9; cf. I, 2, 13, 8 13): everything implies the notion of the existent and cannot exist outside of it.
The most serious attempt to fit him into the history of Western philosophy argues that his notion of barzakh (see section 3.4) offers a viable solution to the problem of defining the indefinable, which has dogged epistemology from the time of Aristotle and led to the despair of modern philosophers like Rorty (Bashier 2004).
