Howard Gardner's framework, cited by Robert Greene alongside Discovering Your Life's Task (Vocation), holds that intelligence is not a single unified trait but comes in several distinct kinds — Greene names intellectual/analytical, musical, pattern, mathematical, social, and kinetic/body-movement — with one type dominating in each person's brain, forming a natural "grain."
Greene uses this as a mechanistic explanation for career misery: choosing a path for external reasons (e.g. going into law purely for money) rather than for fit with your natural intelligence type is why people never reach the goals they actually want. The practical implication is that self-discovery (see Discovering Your Life's Task (Vocation)) should aim to identify which intelligence type dominates before committing to a career, rather than starting from prestige or income and hoping fit follows.
Note: Greene names six types in the interview despite calling the framework "five," likely reflecting loose recall of Gardner's original multiple-intelligences model rather than a deliberate revision.