Copied from the Ayn Rand lexicon, here is a relevant quote from Ayn RAnd.
A given person’s sense of life is hard to identify conceptually, because it is hard to isolate: it is involved in everything about that person, in his every thought, emotion, action, in his every response, in his every choice and value, in his every spontaneous gesture, in his manner of moving, talking, smiling, in the total of his personality. It is that which makes him a “personality.”
Introspectively, one’s own sense of life is experienced as an absolute and an irreducible primary—as that which one never questions, because the thought of questioning it never arises. Extrospectively, the sense of life of another person strikes one as an immediate, yet undefinable, impression—on very short acquaintance—an impression which often feels like certainty, yet is exasperatingly elusive, if one attempts to verify it.
This leads many people to regard a sense of life as the province of some sort of special intuition, as a matter perceivable only by some special, non-rational insight. The exact opposite is true: a sense of life is not an irreducible primary, but a very complex sum; it can be felt, but it cannot be understood, by an automatic reaction; to be understood, it has to be analyzed, identified and verified conceptually. That automatic impression—of oneself or of others—is only a lead; left untranslated, it can be a very deceptive lead. But if and when that intangible impression is supported by and unites with the conscious judgment of one’s mind, the result is the most exultant form of certainty one can ever experience: it is the integration of mind and values.
There are two aspects of man’s existence which are the special province and expression of his sense of life: love and art.
“Philosophy and Sense of Life,” The Romantic Manifesto, 31