Fleeting And Wayward

An sf rationale of telepathy has to explain why ESP is so hard to detect.

Two Rationales

(i) In James Blish's "Nor Iron Bars," parasychology is, possibly, characteristic of the microcosm as electromagnetism is of the macrocosm:

"If so, any telepathic effects that turned up in the macrocosm would be traces only, a leakage or residuum, fleeting and wayward, beyond all hope of control..."
-James Blish, "Nor Iron Bars" IN Blish, Galactic Cluster (London, 1963), pp. 61-92 AT p. 74.

In the sequel:

"Psi forces in general were characteristic only of the subspace in which the primary particles of the atom had their being; their occasional manifestations in the macrocosm were statistical accidents, as weak and indirigible as spontaneous radioactive decay."
-James Blish, "This Earth of Hours" IN Blish, The Best Of James Blish (New York, 1979), pp. 257-280 AT p. 277.

But it turns out that species which think with ganglia instead of with brains are telepathic even across interstellar distances. This would also explain the telepathy of the aquatic micro-organisms in The Seedling Stars.

(ii) In Midsummer Century, Martels speculates that telepathy began as "...a sort of riding light..." (8, p. 61) for the detection of like minds and intentions:

"Such an ability would naturally be selected out in sentient creatures, since from the evolutionary point of view, intelligence would serve the same functions far better. That would leave behind only the maddening vestiges - a sort of vermiform appendix of the mind - which had so persistently disappointed the most sincere occultists from Newton onward." (ibid.)

However, birds' formation flying, migration and homing instinct show that they are telepathic and a future civilization has possibly bred telepathy back into humanity.

Words Used
traces
leakage
residuum
fleeting
wayward
beyond control
statistical accidents
weak
indirigible
vestiges

Martels has a Drang nach Sueden. (8, p. 62)