The Four Investigations, and the Four Resultant True Knowledges

T1606 explains why these four investigations must proceed in exactly this sequence, each building on the conclusion of the one before it. Investigation into name asks first whether the mere collection of names, phrases, and syllables used to designate a dharma has, in itself, any real existence corresponding to its own specific character — since these are all merely provisional, the answer is no. Investigation into the thing asks the same question of the actual referent those names point to — the aggregates, elements, and sense fields themselves — with the same negative result. Having established that neither side of the naming relation, taken alone, is ultimately real, the third and fourth investigations turn to the relation between them: what expresses (the name) and what is expressed (the thing) are mutually dependent for their own intelligibility — a competent speaker who hears only the name is able, through recollection, to understand the referent, and one who grasps only the referent is able, through the same recollection, to supply the name. Investigation into imputed own-nature asks whether, within this very correlation, the dharma's own intrinsic nature amounts to anything beyond what this merely conventional pairing has provisionally established; investigation into imputed distinction asks the same question of every distinguishing feature attributed to it — permanent or impermanent, superior or inferior, having form or formless, visible or invisible, and so on. In every case, the investigation discovers the same thing: what looked like a solid, findable referent turns out, on close examination, to be nothing more than a provisional label resting on a provisional correlation.

T1606 notes simply that each of these four true knowledges is nothing more than the direct fruit of the corresponding investigation just completed — investigation is the diligent search itself, still open to the possibility of finding something; true knowledge is the settled, confirmed result of that search, now knowing with certainty that nothing was there to be found. This fourfold movement — investigate name, thing, imputed nature, and imputed distinction in turn, and arrive at the same unfindability in each case — gives the treatise's own compact method for approaching the deeper teaching, developed at length elsewhere in the Yogācāra corpus, that named things possess no substantial reality of their own apart from the provisional, mutually dependent correlation between language and what language is used to designate.