We do not know what he thought or felt within himself.

On the other hand the method of invisible narration had not at his time acquired the faculty which it possesses now of doing Friday's thinking aloud or exposing fully the workings of his mind.

So that Richardson, whose interests were psychological, whose strength and talent lay in the presentation of the states of mind appropriate to situations of passion or intrigue, had to look about him for a new form, and that form he found in the novel of letters.

In a way, if the end of a novel be the presentation not of action, but of the springs of action; if the external event is in it always of less importance than the emotions which conditioned it, and the emotions which it set working, the novel of letters is the supreme manner for fiction.

Consider the possibilities of it; there is a series of events in which A, B, and C are concerned.
Not only can the outward events be narrated as they appeared to all three separately by means of letters from each to another, or to a fourth party, but the motives of each and the emotions which each experiences as a result of the actions of the others or them all, can be laid bare.