Howard Margolis (1998) Retreating on Blindsight. Psycoloquy: 9(40) Cognitive Illusion (6)

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PSYCOLOQUY (ISSN 1055-0143) is sponsored by the American Psychological Association (APA).
Psycoloquy 9(40): Retreating on Blindsight

Reply to Pearson on Cognitive-Illusion

Howard Margolis
Harris School Public Policy Studies
University of Chicago
1155 E60th Street
Chicago IL 60637
773-702-0926 (fax)


Pearson (1998) may be right after all that the Tycho Illusion could be due to failure of mental rotation imagery rather than imagery blindsight, though it is immediately remedied by doing the cutout and physical rotation. Some misunderstanding would no doubt also have been avoided if the target article had referred to "interpenetration of orbits" instead of "collision."


blindsight, cognitive illusion, mental image, persuasion, psychology of science.
1. In my first reply (Margolis 199b) to Pearson (1998), I argued that the motions needed to account for common intuitions about the Tychonic motions could not be both conscious and credible. I continued to argue that something akin to blindsight with respect to mental imagery seemed to be involved. I would not wholly abandon that conjecture. But on further thought, I see that Pearson is right to argue that it is not needed. Perhaps all that is needed to account for the sense that Tycho's system would be impossible in a world of solid spheres is an inability to imagine how the motions of the system would go. A person's sense is that the system just "won't go" -- in particular, the larger orbits won't go, as stressed in my target article (Margolis 1998a). Contrary to what I said in my earlier reply (Margolis 1998b), this need not be very different from the subjects Pearson describes, who cannot follow an instruction to rotate a D to lie on top of a J well enough to recognize that this would form an image of an umbrella. Where many people can easily do the JD task, no one can spontaneously do the Tychonic task. But even subjects who could not at first do the JD task can easily repeat it once they have seen the rotation done on paper. Similarly, once a person has seen the cutout move, there is ordinarily no longer any difficulty in seeing how Tycho's system moves.

2. While conceding one error, I would like to concede another. In the target article (and in my original reply to Harris and Pearson), I wrote "collision" whenever I needed to make clear that the intersection of orbits which Tycho's scheme requires to match observations was seen (mistakenly, but by the very best experts for four centuries) as intolerable in a world of solid spheres. But using the word "collision" encouraged many readers to suppose that the illusion concerned a direct collision between Mars and the sun, which never was an issue for these experts. The situation was further aggravated by an editing slip (now corrected) which deleted the words "the spheres of" at a critical point in the Rationale I had submitted for the target article. In hindsight, it is apparent that I would have done better to have written of an "interpenetration" of orbits wherever the target article speaks of a "collision" of orbits.


Margolis, H. (1998a) Tycho's Illusion: How It Lasted 400 Years, and What That Implies About Human Cognition PSYCOLOQUY 9 (32)

Margolis, H. (1998b) Tychonic Illusions: Easy Vs. Hard PSYCOLOQUY 9 (38)

Pearson, D. (1998), Imagery Need not be Blind to Fail PSYCOLOQUY 9 (34)

Volume: 9 (next, prev) Issue: 40 (next, prev) Article: 6 (next prev first) Alternate versions: ASCII Summary