. . .too late for it to be any use, I reap the knowledge
Throughout Women of Trachis, Deianira points to the moment of consciousness, the dawning awareness that comes from suffering, then from reflecting upon the experience. She tells the chorus:
You have heard of my trouble, I would guess, and that has brought you here. But the anguish which consumes my heart—may you never come to know it through your own experience!—to that now you are total strangers. (141-144)First she distinguishes between hearing of trouble (in this case, hers) and actually experiencing it for oneself. This difference between knowing-through-hearing and direct experience is the difference between being a young girl and being a wife:
Yes, a young life grows in those sheltered regions of its own, and the Sun-god's heat disturbs it not, nor rain, nor any wind. Rather it takes up a toilless existence amidst pleasure, until such time as she is called “wife” instead of “maiden”, and takes her portion of anxious thoughts in the night, fearing for husband or for children. At that point a woman could understand the burden of my misfortunes by recalling [σκοπῶν: looking, considering] her own experience. (144-150)One's own experience sheds light upon that which one hears about from others, enabling understanding.
It's precisely that light of experienced understanding that is eclipsed in charms. The "precepts" of Nessus, carved in her memory, banish all light:
I neglected no part of the precepts which the savage Centaur gave me when he was hurting from the bitter barb in his side; they were in my memory, like the graven words which no hand may wash from a tablet of bronze. Now this was his order to me, and I obeyed it:  to keep this potion hidden in inmost recesses always away from fire and untouched by the sun's warm ray, until I should apply it, newly spread, where I wished. So I had done. And just now, when the moment for action had come, I performed the anointing secretly in the rooms of the house  with a tuft of soft wool which I had plucked from a sheep of our home-flock; then I folded up my gift, and laid it, unvisited by sunlight, within its hollow chest, as you saw.Just previously she had spoken of the tablet carved with strange signs that Heracles had shown her. The signs could not be read, at least by her - they are dark, but they contain the fate of Heracles. But after she's finished composing the charm in perfect darkness, an accident brings light, and something unexpected:
But as I am going back into the house, I see a thing inexplicable by words and beyond the knowledge of the human mind to understand.  For somehow I happened to have thrown the ball of wool, with which I had been anointing the robe, into the full blaze of the sun's rays. As it grew warm, it ran all into confusion, and quickly crumbled to powder on the ground, like nothing in appearance so much as  the morsels left by a saw's teeth when wood is cut. It lies just so, fallen. And from the earth, where it lay exposed, clotted foam seethes up, like the rich juice of the blue fruit from the vine of Bacchus when it is poured on the ground. (680-704)This event - much like a chemical reaction in an experiment - triggers thinking:
And so I am distraught, and I do not know to which side my thoughts should fall. I only see [ὁρῶ] that I have brought a terrible deed to completion. For why or in thanks for what should the monster in his death-throes have shown good will to me, because of whom he was dying? Impossible! No, he was enchanting me in order to destroy the man who had shot him.Into the dark spell of the charm-making stabs a ray of sunlight, and immediately, because Deianira is no fool, she sees what she has been doing in an entirely other "light." The script of the charm was precisely like those inscrutable symbols on Heracles' tablet, until the sun not only brings it to light, but acts upon the tuft of wool. In an instant, her mind is changed, the illusion is consumed in a flash of insight. Then come the words printed above:
And now too late I gain the knowledge of this, when it can no longer help.Deianira sees that she was blind when she acted, but that doesn't change the act. It is "too late": the lethal effect of the deed executed in ignorance cannot be reversed in the throes of consciousness, although its meaning can be, and is.
No clearer example of the tragic divorce of power from knowledge, action from thought, is necessary. Sophocles is never more Sophoclean than when this divorce is what is on offer on his stage. He wishes us, epigones of both Chiron and Nessus, of divine logos and dark magic, to "see" that while we are creatures that know and creatures that do, it is perfectly possible that we have no understanding of what we are doing, and no power to do anything about what we've done once we understand, too late, what it was.
Later on we learn that Heracles wrote down the prophecies he heard at Dodona:
It was foreshown to me by my father far in the past that I would perish by no creature that had the breath of life, but by one already dead, a dweller with Hades. So this savage Centaur in death has killed me alive, just as the divine will had been foretold. And I will show you how later oracles tally with the first and testify to the old prophecy. I wrote them down [εἰσεγραψάμην] for myself from the mouth of my father's oak of many tongues [πολυγλώσσου] in the grove of the Selli, who dwell on the hills and sleep on the ground. (1159-69)Can Zeus's "oak of many tongues" in fact be unambiguously translated into unequivocal writing? It's one thing to write down what one hears, another to know how to read what's written. By the time Heracles knows what his writing says, it is, once again, too late.
The Centaur who kills Heracles is dead, but his precepts, "like the graven words which no hand may wash from a tablet of bronze," live on. Automata working beyond the end of the tongue that spoke them, Nessus's words charm beyond the grave.
Both Deianira and Heracles discover that they had utterly misread signs that they were given - signs from Zeus in Heracles' case; signs from Nessus for Deianira. The pattern beautifully described by Deianira of the young girl who must experience suffering before she can understand life holds: we act, then suffer discovering what we were doing, or what was being done to us.
The joint errors of Deianira and Heracles bring the play to a climactic impasse. What can Heracles do, other than die? Yet the play doesn't end quite that way. He acts again before his end. The effect of that remains to be seen.