 Another class of potential problem cases are vacuous terms. So, for example, people can think about unicorns, fountains of youth, or the planet Vulcan. Cases such as these are discussed in Stampe (1977) and Fodor (1990a), among other places. These things would be physical objects were they to exist, but they do not, so one cannot causally interact with them. In principle, one could say that thoughts about such things are not counterexamples to causal theories, since causal theories are meant only to offer sufficiency conditions for meaning. But, this in principle reply appears to be ad hoc. It is not warranted, for example, by the fact that these excluded meanings involve abstract objects. There are, however, a number of options that might be explored here.