§ 327. _Processes of necessity as opposed to processes of habit._--The combinations -pd, -fd, -kd, -sd, and some others, are unpronounceable. Hence words like _step_, _quaff_, _back_, _kiss_, &c., take after them the sound of -t; _stept_, _quafft_, &c., being their præterites, instead of _stepd_, _quaffd_. Here the change from -d to -t is a matter of necessity. It is not so with words like _weep_, and _wept_, &c. Here the change of vowel is not necessary. _Weept_ might have been said if the habit of the language had permitted. A definition of the word _irregular_ might be so framed as to include all words whose natural form was modified by any euphonic process whatever. In this case _stept_ (modified by a process of necessity), and _wept_ (modified by a process of habit), would be equally irregular. A less limited definition might account words regular as long as the process by which they are deflected from their natural form was a process of necessity. Those, however, which were modified by a process of habit it would class with the irregulars. Definitions thus limited arise from ignorance of euphonic processes, or rather from an ignorance of the generality of their operation. § 328. _Ordinary processes as opposed to extraordinary processes._--The whole scheme of language is analogical. A new word introduced into a language takes the forms of its cases or tenses, &c., from the forms of the cases or tenses, &c., of the old words. The analogy is extended. Now few forms (if any) are so unique as not to have some others corresponding with them; and few processes of change are so unique as not to affect more words than one. The forms _wept_, and _slept_, correspond with each other. They are brought about by the same process: viz., by the shortening of the vowel in _weep_ and _sleep_. The analogy of _weep_ is extended to _sleep_, and _vice versâ_. Changing our expression, a common influence affects both words. The alteration itself is the leading fact. The extent of its influence is an instrument of classification. When processes affect a considerable number of words, they may be called _ordinary_ processes; as opposed to _extraordinary_ processes, which affect one or few words. When a word stands by itself, with no other corresponding to it, we confess our ignorance, and say that it is affected by an extraordinary process, by a process peculiar to itself, or by a process to which we know nothing similar. A definition of the word _irregular_ might be so framed as to include all words affected by extraordinary processes; the rest being considered regular. § 329. _Positive processes as opposed to ambiguous processes._--The words _wept_ and _slept_ are similarly affected. Each is changed from _weep_ and _sleep_ respectively; and we know that the process which affects the one is the process that affects the other also. Here there is a positive process.