The Tabu (The Embargo)
The prohibition--largely unstated but almost always assumed in our scientific age--that the natural world is to be understood without resorting to any hypotheses involving concepts of mind or subjectivity or spirit. "I am not . . . saying," Barfield explains in Speaker's Meaning, "that there is any embargo on a man's arriving at, and even expounding, some kind of idealist philosophy or other, some abstract theory or rational demonstration that mind must be conceived as anterior to matter--Berkeleyism, Kantianism, neo-Kantianism, Bergsonism, Buddhism--there is a wide choice." The tabu is of a different nature.

What the tabu enjoins is that he shall put all that out of his mind as soon as he lays down his pen or leaves the lecture room; above all, that he shall not attempt to apply it in any other realm of inquiry that he may enter, an inquiry, for instance into the origin of language, or of myth, or (if he happens to be a scientist) into the basis of post-Cartesian scientific method. As soon as he goes on to do this, he is bound as a respectable member of educated society to start off from a whole array of presuppositions that are quite incompatible with the conclusions he has arrived at as a philosopher or physicist, or whatever. (109-110)

The tabu or the embargo (Barfield uses the words almost interchangeably) results in a drastic curtailment of modern science's claim on Truth:
    The limited scope of all scientific inquiry is today often emphasized rather strongly by those engaged in it. So much so, that when we have heard them on the subject, we are sometimes left with the feeling that we ought to look on all scientific theories as mere "hypotheses" in the sense of the Platonic and medieval astronomers, and that it is wrong to take any of them with the "literalness" that embroiled Galileo with the Church. They are, at best, we are assured, the mathematical formulae which up to the time of writing have been found the simplest and most convenient for--well, for saving the appearances. In physics, in particular, there is a marked tendency to treat almost as an enfant terrible anyone who takes the models literally enough to refer to them in any context outside that of physical inquiry itself. (SA 54)
Under the sway of the tabu "those who [a century ago] insisted that organic forces specifically different from mechanical ones were accused of 'Vitalism,'" and "those who today insist they are different from both mechanical and electromagnetic forces are commonly accused of 'Mentalism.' That "the absolute 'refusal of most biologists to admit the existence of any forces remotely resembling mental operations" necessitates the backdoor adoption of a "picturesque vocabulary" to account for "operations which obviously are quite crudely mental,"1 we seem, under the tabu, not even to notice the contradictions in our thought (xx 129).

Outside the sciences, the tabu produces other, more pernicious effects, explained by the Meggid in Unancestral Voice:

The tabu bars all approach to an awareness of the encompassing spirit that persists and sustains through the transformation that is waking and sleeping and through the transformation that is life and death. It persuades the mind that the borderland between the non-spatial and the spatial manifestations of spirit cannot and should not be broached by the understanding. Its foundation was deliberately laid . . . at a time when the form of Western thought was itself yet young and delicate. And as the twig was bent, the tree has grown. In the course of the centuries, as the forms of Western thought have strengthened and hardened, the barrier has been entrenched and fortified by the two adversaries, till today it has become a tabu. Even to think of crossing it is indecent. It takes courage to disregard a tabu. . . . The scientist bows before it, because the whole of his science is founded on it; the philosopher because he has taken his cue from science and would now rather eliminate the ghost than sacrifice the machine; the religious because, for, him, God must be God the Paterfamilias or nothing; the artist--nowhere perhaps than here has the strength of the tabu shown itself more plainly. (UV 108-109)

See in particular Unancestral Voice, passim, Worlds Apart, passim.
1Barfield provides examples: "I mean, for example, when we hear strictly orthodox geneticists talking of codes and blueprints and assuring us that each growing organism refers to the blueprint for instructions, and then of constraints imposed by nature, or mistakes in following the instructions and so forth" (xx 129).