Allowing knowledge that goes beyond the information given in sense experience raises two difficult philosophical questions. The first concerns how concepts can be meaningful if they go beyond experience, which empiricists take to be the source of meaning. The answer arises from the recognition that the meaning of concepts, construed as patterns of neural activation, is relational and multidimensional. Theoretical concepts like atom and virus are only indirectly related to sense experience, but that is neither a philosophical nor a psychological problem because such concepts are richly related to other concepts by virtue of the theories of which they are a part. For example, the concept atom is related to other concepts, such as element, molecule, electron, and proton, all of which contribute to the atomic theory of matter that explains a vast number of experimental findings in physics, chemistry, and molecular biology. The meaningfulness of such concepts is a puzzle only if one assumes a narrowly empiricist view of how concepts depend on sense experience.