Thom's Principle: rich structures are more numerous in low dimension

One possible reason is that in higher dimensions there are more degrees of freedom that can be used to unravel and untangle things, which often leads to simpler structures. This reason has particularly been used as an explanation as to why the geometry/topology of high-dimensional manifolds is sometimes easier to deal with than that of lower dimensional ones. The canonical illustration that accompanies the previous sentence is the failure of the Whitney trick in dim < 5; the fact that the trick holds in higher dimensions was, for example, instrumental in Smale's proof of the $h$-cobordism theorem.

Pro

I think the examples given are instances of Guy's "strong law of small numbers". That seems at least poetic reason for low-dimensional specializations of your favorite theory to be different in character from high-dimensional specializations.

Con

An example of increasing richness indicating Thom was thinking about something else:

• "The" connected 0-manifold is a point
• "The" connected compact 1-manifold is a circle
• Connected compact 2-manifolds are connect sums of tori or of projective planes; they are uniformizable.
• Connected compact smooth 3-manifolds are piecewise geometrizable, where the joints are among spheres and tori.
• Connected compact $(3+n)$-manifolds "solve" the word problem for groups; particularly weird: there are smooth examples that have contractible stable closed periodic geodesics in any smooth metric.

This is sort-of what I'd call rigidity in low dimensions, and richness in high dimensions. Perhaps the theories in low dimension are richer in the sense that there are more universal statements we can prove, but there seems to be a greater wealth of useful examples in higher dimension.

Whether this is an instance of Thom's principle as quoted or an exception, it is still an instance of Guy's law, in that the low-dimensional behaviour isn't representative of high-dimensional behaviour.

I am reminded of the fact that for geometries of dimension strictly higher than 2, the Desarguesian and Pappian configurations are equivalent, so that (in my view) there is a greater degree of uniformity in higher dimensional geometric spaces. I think Thom's principle (as noted in another post) is a selection principle and tells the reader what Thom thinks of by rich structure. If structures are essentially (in mathematical logic, some notion of conservative) extensions of incidence relations, then I am not too surprised that something like this principle should hold, since a certain amount of uniqueness is lost in a structure that should be invariant under more geometric transformations. I am not a geometer, though, so go ask someone like Hrushovksii what they think (from a mathematical logic perspective) of Thom's principle.

Gerhard "Ask Me About System Design" Paseman, 2011.09.13