A philosophy subsumes principles. In taijiquan there are abundant principles to fill many inch-thick books published. All these principles tie-in with the philosophy of taiji. Taiji is the draw-string of a net of principles. To talk about principles without touching on philosophy makes for diverse beliefs and different schools; to discuss philosophy alone leaves one no concrete footing. Both subjects should be broached. A common Chinese saying: [On certain tenuous subjects] "One can only grasp its meaning but not by words". However, we must rely on words and merge toward the word/meta-lingua zone.
Taiji is the dyad of changing yin/yang. Everything material and immaterial can be dichotomized: dark/light, soft/hard, cold/hot, still/move, black/white, female/male, moon/sunand these must have the change factor to be regarded as yin/yang in taiji: dark changing to light, soft changing to hard, cold changing to hot, still changing to moveand vice versa. Among prominent concepts in taiji include integral, cyclic, binary subdivision, meaning, respectively, that taiji is the integral (synthesis) of yin/yang, that yin/yang interact cyclically [as the taiji symbol suggests], that within yin/yang each may also have binary yin/yang, ad infinitum. [Yi Jing, the Book of Changes, is the resource on yin/yang.]
Highfalutin as this may sound, taijiquan constitutes taiji in its "quan" [boxing, pugilism]. Taijiquan (TQ) concerns with the changing yin/yang of soft/hard, firm/light, still/move, combine/divide, defence/offence. For instance, with left guard, thrust right. A one sided maneuver, a lapse, of course, falls short of taiji. Beware: what goes around comes around.
Practitioners in their modus operandi embrace taiji mentally and physically. Some adherents consider themselves devotees and live taijiquan. (For devotees, taiji=taijiquan for short when there is no confusion). When one is in that realm, he or she becomes a taijiquaner, a philosopher. A practitioner, philosopher or not, who takes in disciples can become a master, an honorific title conferred, never self-styled.
With a philosophy of taiji in mind, incomplete though it may be, the learner can proceed to begin taijiquan, knowing one's knowledge on the subject will accrue. Right from the opening stance you are a taiji microcosm: sink from the hip to your feet, that's yin; with erect spine and neck lift your spirit to the top of your head, that's yang. This model shall apply throughout the exercise. In transition or change one hand acts yin, the other yang, vice versa. Do this habitually and the details will fall into place.
A thumbnail sketch: taijiquan, its form some call it, or the routine exercise, goes thru the same bound moves each time, the moves put in a logical, interesting sequence. The sequence consists of 37 basic moves that are repeatedly exercisedthe important repeated more often than the lesserlinked in between by moving sideways/forwards/backwards.