 Many predicates in scientific theories (e.g., “being a gene” and “being a belief”) are functionally defined. Namely, their meaning is fixed by appealing to some function or set of functions (e.g., encoding and transmitting genetic information). To make sense of functions, properties are needed. First, functions can be thought of as webs of causal relationships between properties or property-instances. Secondly, predicates such as “being a gene” may be taken to refer to higher-level properties, or at least to something that has such properties. In this case, the property of being a gene would be the higher-level property of possessing some further properties (e.g., biochemical ones) that play the relevant functions (i.e., encoding and transmitting genetic information). Alternatively, being a gene would be a property that satisfies the former higher-level property. Thirdly, the ensuing realization relationships between higher-level and function-playing properties can only be defined by appealing to properties (see entry on  functionalism).