In The Philosophy of Grammar Jespersen argues that “sane logic” is one of the three bases of his description of the grammar of English, the other two being “sound psychology” and “solid facts of linguistic history.” Balancing those three bases is essential. He also said that “logic is of the greatest value for the building up of our grammatical system and for the formulation of our grammatical rules or laws.” But he did not feel that we should use a cold, formal logic to condemn new developments in English speech: “Instead of that, we should cultivate a broader-minded logic which would recognize, for instance, that from the logical point of view the indirect object may be made the subject of a passive sentence just as much as the direct object,” as in “He was offered a crown.” He said that “we should beware of calling in a superficial logic to condemn what in a more penetrating consideration may appear perfectly justifiiable.
In the Growth and Structure of the English Language he said that: “No language is logical in every respect, and we must not expect usage to be guided always by strictly logical principles. It was a frequent error with the older grammarians that whenever the actual grammar of a language did not seem conformable to the rules of abstract logic they blamed the language and wanted to correct it.” These older grammarians, he believed, granted abstract logic an undeserved hegemony.
On the other hand, Jespersen greatly admired the fundamental logic of the English language and argued that other than Chinese, “there is perhaps no language in the civilized world” that is as logical as English. He cited, for instance, the precision and consistency of our verb tense system, with its perfect and progressive tenses. But he also admired the way that, in spite of its fundamental logicality, English did not apply the rules of logic in a pedantic and stifling way. For example, he pointed to the flexibility of our use of number – as in the way a word like team can be treated as either singular or plural, depending on whether the speaker intends to stress the team’s unity or plurality. To treat this flexible logic in the phrase and clause structure of English, he developed his theory of the “the three ranks.” In this analysis, which is essentially a scheme of logical subordination and is meant to work with the standard parts of speech, he classified words as primaries, secondaries, or tertiaries. A primary is the word in a combination to which the other words are subordinate. Secondaries modify or define primaries, and tertiaries modify or define secondaries. For instance, in a phrase like “extremely hot weather” weather is the primary; hot is a secondary, modifying weather; and extremely is a tertiary, modifying hot. (See chapter seven of The Philosophy of Grammar for more details.)
All in all, Jespersen concluded that “The English language is a methodological, energetic, business-like and sober language, that does not care much for finery and elegance, but does care for logical consistency and is opposed to any attempt to narrow-in life by police regulations and strict rules either of grammar or of lexicon.”
Jespersen was born one year after the publication of Darwin’s Origin of the Species, and his approach to English grammar reflects his sense of language as an evolving, changing system, adaptive to the needs and will of its human users, and not as some sort of unchanging system in which any new development is seen as corruption. While biological systems evolve via adaption and selection driven largely by changes in the surrounding world, language evolves via adaption and selection driven by the needs and will of its human users. Jespersen felt that the English language was a living system, to be judged against the needs of its users and the changing world to which it is used to refer rather than against some abstract, unchanging logic. His was, as he said, a sane logic.