The preposition ‘to’ is not an essential part of the infinitive, for it is not an invariable sign of it. Many verbs, especially the auxiliaries, are followed by the infinitive without ‘to.’ The infinitive in Old English ended in-an and did not have ‘to’ before it. It was treated as a declinable abstract-noun, and a dative form (called the gerund) was used with the preposition ‘to’ to denote purpose, as ‘He that hath ears _to hear_’; to hear = _to gehyranne_. This gerundive infinitive passed into modern English with the loss of the dative ending. From denoting purpose, the ‘to’ came to denote the ground of an action, and may indicate the cause or condition of an action. Thus the gerund (the infinitive with ‘to’) came to be used in place of the simple infinitive. The preposition ‘to’ has in this manner come to be nothing but an inflection for the nominative and accusative cases, and to lose the notional idea of a preposition except with the infinitive of purpose.
The infinitive is a verb-noun. The authority for this begins with the fact that it was in Old English declined as an abstract-noun. Prof. Whitney, of Yale, says that the infinitive is a verbal-noun expressing in noun-form the action or condition which the verb asserts. Welsh also calls it a verbal-noun. Both of them say that mood serves the purpose of showing a difference in the mode or manner of assertion. These two authors do not consider that the difference in the manner in which an infinitive and the indicative, subjunctive and imperative express action is of the same nature as the difference between the indicative and subjunctive, or between the subjunctive and imperative. These differences, as any one can plainly see, are altogether unlike. The distinction between the infinitive and the three moods is by far greater than any distinctions that the moods have among themselves. The infinitive, gerund and participle have their verb natures alike. All three are unlimited to person, number, or order of time. It is only in the other half of the compound that they differ. The gerund partakes of the nature of a verb and a noun, while the participle partakes of the nature of a verb and an adjective, and the infinitive is verbal in the dependence of other words upon it and has the nature of a noun in its dependence upon other words. From this it is seen that, if the term ‘mood’ can be applied to the infinitive, as it pertains only to its verbal nature, it can be applied to participles and gerunds. If all three are called moods, then there is a nomenclature which is not needed and still not such as will separate the two great classes of verb-forms (finite and infinite) and show the difference in their expression of action, being or state. The name ‘infinitive,’ it is true, would denote its manner of expression, but the participle and the gerund have nothing in their names to denote this same quality. What is needed is a distinction between the two great classes. If the infinitive is not called a mood, there is this distinction; if it is called a mood, then there is need of some other names by which to distinguish the two classes.