Formally, you don't. Informally, you might try approximate definitions and see how they fail to capture elements of reality, or you might try and find analogies to other situations that have been modeled well and try to capture similar structure. Mathematicians et al usually don't start new fields of inquiry from a set of definitions, they start from an intuition grounded in reality and previously discovered mathematics and iterate until the field takes shape. Although I'm not a physicist, the possibly incorrect story I've heard is that Feynman path integrals are a great example of this.