 Of course, in the case of intentionality, unlike the case of water, the hidden nature or essence cannot be an inner structure, if a teleological theory is correct. On such a theory, intentionality is alleged to be an historical kind, so the previously hidden nature is alleged to be a matter of history. As proponents of teleological theories point out, there is an apparent need for other historical kinds in biology (e.g., offspring, homologs and   species).[21] 