 A psychophysical product (e.g., a speech) differs from a mental product (e.g., a judgment) because it is perceptible to senses; it differs from a physical product (e.g., a run) because in the psychophysical action which produces it (speaking) a mental action is also involved, which has bearing on the physical action, and thus on its product (§10). In some cases, a psychophysical product expresses a mental product; for instance, a sentence is a psychophysical product which expresses a mental product, a judgment.