93. Anisotropy, noun. From Greek an, “not,” + iso, “equal,” + tropos, “a turning.” The quality of being anisotropic.

An anisotropic property is one whose magnitude varies depending on the direction in which it is measured. A great many objects, materials, and substances exhibit anisotropy. For example, the wood fibers in paper and the collagen fibers in parchment or vellum have much more tensile strength when pulled in the direction of their longitudinal alignment and much less when pulled against it. This is why manuscripts—even if carefully stretched on a frame at the beginning of their lives—tend to curl and undulate in predictable ways over time.

The brain, also made of bundles of fibers, is an anisotropic environment. Water molecules, for instance, diffuse more quickly in certain directions than others, depending on the barriers they encounter. This is the phenomenon that has let us see our brains more clearly than we ever have before—and in the process raised some old familiar questions about who we really are.

This matter that we’re carrying isn’t one thing and the same—it has strong inclinations. Some truisms deserve their names. So much depends on how you look at it.


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