However, even in the intra-modal case, the region of non-detection defined by the magenta points is only extended slightly away from the centre along the cues-discordant diagonal, whereas the mandatory fusion theory predicts that it should extend along an infinite metameric band.
Now, one line of response might be to say that the doubts the skeptic raises over our modal knowledge here can themselves be blocked or shown to be spurious, for example, by providing evidence for the reliability of our methods in the modal case, or questioning the right of the skeptic to use the mere possibility of error against such knowledge.
It seems that I can adequately answer the parallel question in the modal case by saying that I am stipulating that, when I say that there is some possible world in which Russell is a playwright, the relevant individual in the possible world (if there is one) is Russell (and not some other potential or actual playwright).
So why should things be different in the modal case?
However, it seems that exactly similar moves are available in the modal case to accommodate 'change' of properties across possible worlds.
ii) Particularly in the intra-modal case (Fig. 4b), the observed experimental performance is significantly worse in the cues discordant quadrants (2&4) than predicted by any of the basic theories (1, 2, and 4) which allow detection based on individual cues (magenta points are outside of the red lines in Fig. 4(b), quadrants 2&4).
In particular, the classical theory of ideal observer maximum likelihood combination which Hillis et al. concluded applied in the within-modal case retains a strong qualitative discrepancy with the experimental results (Fig. 4b, green lines and points).
The primary research question was: if some individuals have large within-modality AB magnitudes, might they also show an AB in the cross-modal case?
That result generalizes easily to the poly-modal case (van Benthem 1996, p. 88).
Since the poor performance in the cues discordant quadrants 2&4 was noted to be less prominent in the inter-modal case (Fig. 4a), Hillis et al. concluded that mandatory fusion applied within (Fig. 4b) but not between (Fig. 4a) the senses [17].
