'Lewis'; the relative is non-defining; and the clause is (not merely contributes to) a comment.
b. 'That' and 'who' or 'which'.
'That' is evidently regarded by many writers as nothing more than an ornamental variation for 'who' and 'which', to be used, not indeed immoderately, but quite without discrimination. The opinion is excusable; it is not easy to draw any distinction that is at all consistently supported by usage. There was formerly a tendency to use 'that' for everything: the tendency now is to use 'who' and 'which' for everything. 'That', from disuse, has begun to acquire an archaic flavour, which with some authors is a recommendation. De Quincey, for one, must certainly have held that in exalted prose 'that', in all connexions, was the more dignified relative; his higher flights abound in curious uses of the word, some instances of which are quoted below.
This confusion is to be regretted; for although no distinction can be authoritatively drawn between the two relatives, an obvious one presents itself. The few limitations on 'that' and 'who' about which every one is agreed all point to 'that' as the defining relative, 'who' or 'which' as the non-defining. We cannot say 'My father, that left Berlin last night, will shortly arrive', and an examination of instances would show that we can never use 'that' where the clause is unmistakably non-defining. On the other hand, we cannot say 'All which I can do is useless'; this time, it is true, the generalization will not hold; 'which' can, and sometimes must, be used, and 'who' commonly is used, in defining clauses. But that is explained partly by the obvious inconvenience sometimes attending the use of 'that', and partly by the general tendency to exclude it from regular use, which has already resulted in making it seem archaic when used of persons, except in certain formulae.
The rules given below are a modification of this principle, that 'that' is the defining, 'who' or 'which' the non-defining
