Vallée's opposition to the ETH theory is summarised in his paper, "Five Arguments Against the Extraterrestrial Origin of Unidentified Flying Objects," Journal of Scientific Exploration, 1990:

"Scientific opinion has generally followed public opinion in the belief that unidentified flying objects either do not exist (the "natural phenomena hypothesis") or, if they do, must represent evidence of a visitation by some advanced race of space travellers (the extraterrestrial hypothesis or "ETH"). It is the view of the author that research on UFOs need not be restricted to these two alternatives. On the contrary, the accumulated data base exhibits several patterns tending to indicate that UFOs are real, represent a previously unrecognized phenomenon, and that the facts do not support the common concept of "space visitors." Five specific arguments articulated here contradict the ETH:

unexplained close encounters are far more numerous than required for any physical survey of the earth;

the humanoid body structure of the alleged "aliens" is not likely to have originated on another planet and is not biologically adapted to space travel;

the reported behavior in thousands of abduction reports contradicts the hypothesis of genetic or scientific experimentation on humans by an advanced race;

the extension of the phenomenon throughout recorded human history demonstrates that UFOs are not a contemporary phenomenon; and

the apparent ability of UFOs to manipulate space and time suggests radically different and richer alternatives."