Participation was a technical term for the Schoolmen of the Middle Ages. The latter, in turn, inherited it along with the rich philosophical lineage that flowed like a river of wisdom through Plato, Aristotle, and the New Testament and Neo-Platonist writers like Paul, Proclus, Boethius, and Dionysius the Areopagite. These philosophers wrote in Greek so they expressed the notion of participation by terms like “metalepsis” (μετάληψις), “methexis” (μέθεξις), or sometimes “koinonia (κοινωνία). To understand the notion of participation, we can turn to Aquinas, who defines the term in the preface to his commentary on Boethius’ De hebdomadibus. Aquinas identifies three genera of participation:
(1) A particular takes part of an universal. He offers the following examples of this genus of participation: man participates animal, and Socrates participates man.
(2) Similarly, a subject participates its accident and matter participates form. Aquinas does not feel it necessary to offer any examples of this second genus, but we could borrow an idiosyncratic example from Aristotle and say that Socrates participates “snub-nosedness” because he, as a subject, had a snub-nose, as an accident. Perhaps it was an inside joke to the Ancient Athenians. Similarly, blood and bones participate Socrates because Socrates is the form of blood and bones and the matter of the respective form. These are examples of subject-accident and matter-form relations, respectively.
(3) Also, an effect participates its cause. Barfield specifically cites the example that Aquinas offers in this case, who writes: “Suppose we say that air participates the light of the sun, because it does not receive it in that clarity in which it is in the sun.”  The sun being the cause of the light, the air participates that cause.
Every particular participates its universal, every effect participates its cause, all matter participates its form. In the same manner, all beings participate Being (plura entia, sed non plus entis). Similarly, all things that were made participate the Lógos (Λóγος). This provides the rainbow-bridge to Barfield’s notion of “Final Participation.”
Final participation implies the recognition of our own participation of the Lógos. To participate the creative and organising principle of the cosmos—that by which all things were made—however, is simultaneously to be participated by all things that were made. Procession and reversion are united. “Is it not written in your law, I have said, Ye are gods?” This is quite a lofty conclusion so let us come at it by a different to ensure we have not attained this height by some trickery or deception.
Consider that there is no such thing as an unperceived phenomenon. Indeed phenomenon literally means “what shows itself,” or “what appears” (φαινόμενον). It is hard for us to think this way today because we are bewitched by Kant and Einstein into thinking that real reality must be mathematised and imperceptible, hidden behind a veil of philosophical quiddities or differential equations. But let us imagine Einstein recapitulating Newton’s notorious experiment with falling fruit. If Einstein, after developing his theory of General Relativity in 1916, were to watch an apple fall, he did not see, in the mode of Aristotle and the natural philosopher of old, the Earth element striving to rejoin its Mother. Nor did he see the operation of universal gravitation, in the manner of Newton. Instead, Einstein would have perceived a demonstration of warped spacetime around a massive body. Obviously he didn’t see this with his eyes. Still this is hardly an objection, since we really don’t see any thing with our eyes. Maybe we see colours with the eyes alone, but it is already questionable whether the sight of colour does not imply a conceptual element as a necessary condition for its perception. Thus, the fact that Einstein perceived the phenomenon with his mind does not imply that he did not perceive it.
Likely few people would contest the above, and yet fewer still think through its implications. Barfield, however, was an exceptional thinker. From the conclusion above, the former arrived at his notion of Final Participation. Given that the mind is already operative in even the most rudimentary perceptual acts, the mind can awaken to this activity. This changes the world. “This changes the world” is not a cliché or a figure of speech, but a literal statement of fact. Awakening to the our own creative activity in perception changes the world because the world phenomenal. We don’t see “the world” with our eyes any more than we see other things. Rather we perceive the world according to how we conceive of it. And if we assume this mantle of responsibility, once born by the gods of old, then we participate finally the same Lógos which our ancestors participated originally.
 Cf. Physics, Book II.
…et similiter effectus dicitur participare suam causam, et praecipue quando non adaequat virtutem suae causae; puta, si dicamus quod aer participat lucem solis, quia non recipit eam in ea claritate qua est in sole.