Figure 11.2. Phototropism in a Phycomyces sporangiophore: a multiple-expo-
sure photograph showing upward elongation of the sporangiophore, followed
by phototropic elongation toward light coming from the left (one exposure
every 3 minutes).
What can we say about our experience of the world in the context
of the present discussion of sensory perception? It is conventional
to begin by assuming there is a world—a physical reality—existing
independent of our interacting with it. Note that this, too, is an
assumption, or at least a working hypothesis, because everything we
know comes to us by way of our perception and consciousness. How-
ever, if we assume the existence of some kind of “external” physical
world, then our experience of that world depends on what is actually
there—the physical things that are out there, what actually exists. In
philosophy this is the realm of ontology: the study of the nature of re-