[…] the phonological system of various languages may differ from each other in two ways. They usually differ as to the number of phonemes and the phonetic characteristics of each, but they also often differ as to the arrangements in which the phonemes occur relative to each other.
- Hockett (1958), a pag.84

the ‘phonological’ system: a stock of phonemes, and the arrangements in which they occur;
- Hockett (1958), a pag.137

The phonological system of a language is therefore not so much a “set of sounds” as it is a ‘network of differences between sounds’. In this frame of reference, the elements of a phonological system cannot be defined positively in terms of what they “are”, but only negatively in terms of what they are not, what they contrast with.
- Hockett (1958), a pag.24-25