David Corfield Gilbert Ryle

When a sentence is (not true or false but) nonsensical or absurd, although its vocabulary is conventional and its grammatical construction is regular, we say that it is absurd because at least one ingredient expression in it is not of the right type to be coupled or to be coupled in that way with the other ingredient expression or expressions in it. Such sentences, we may say, commit type-trespasses or break type-rules. Latterly the attention of logicians has been focused on certain sorts of type-trespasses, like those which are committed by ‘I am now lying’ and ‘“heterological” is heterological’. These sorts are interesting, because their absurdities are not obvious but manifest themselves in the generation of contradictions or vicious circles, whereas ‘Saturday is in bed’ is obviously absurd before any contradictions are seen to result from the hypothesis that it is true.

Moreover, we can be actually led by seemingly valid arguments to propounding propositions of the former sorts, whereas only the deliberate intention to produce balderdash would get us to formulate sentences of the latter sort. That is, some type-trespasses are insidious and others are not. It is the insidious ones which force us to consider type-rules; the others we only attend to because we are already considering type-rules. But it would be a mistake to restrict the theory of types to the theory of certain special type-rules.

To ask the question To what type or category does so-and-so belong? is to ask In what sorts of true or false propositions and in what positions in them can so-and-so enter? Or, to put it semantically, it is to ask In what sorts of non-absurd sentences and in what positions in them can the expression ‘so and so’ enter? and, conversely, What sorts of sentences would be rendered absurd by the substitution for one of their sentence-factors of the expression ‘so and so’? I adopt the word ‘absurd’ in preference to ‘nonsensical’ or ‘meaningless’ for the reason that both the two last words are sometimes used for noises like ‘brillig’ and ‘abracadabra’, and sometimes for collocations of words having no regular grammatical construction. Moreover, both have recently been adopted for polemical purposes in aid of a special theory. ‘Absurd’ has helpful associations with the reductio ad absurdum, and even its nuance of ridiculousness is useful rather than the reverse, for so many jokes are in fact type-pranks. (Categories, Collected papers Vol. 2 p. 188)

Later

It has long been known that what a proposition implies, it implies in virtue of its form. The same is true of what it is compatible and incompatible with. Let us give the label “ liaisons ” to all the logical relations of a proposition, namely what it implies, what it is implied by, what it is compatible with and what it is incompatible with. Now, any respect in which two propositions differ in form will be reflected in differences in their liaisons. So two propositions which are formally similar in all respects save that one factor in one is different in type from a partially corresponding factor in the other, will have liaisons which are correspondingly dissimilar. Indeed the liaisons of a proposition do not merely reflect the formal properties of the proposition and, what this involves, those of all its factors. In a certain sense, they are the same thing. To know all about its liaisons is to know all about the formal structure of the proposition, and vice versa. Though I can obviously entertain or believe a proposition without having yet noticed all its liaisons. Indeed I must grasp it before I can consider them, otherwise I could not be the victim of antinomies. (Categories, pp. 204-205, pp. 191-192 of Collected Papers Vol 2)

Continues by suggesting logical form is not enough, cf. Witt and colours, but a richer logic?