The "perplexities" that threatened to retard the healing of Uncle Toby's wound consisted of the difficulty of explaining clearly the technical details of where and how he received the wound; he would "oft times puzzle his visiters, and sometimes himself too." He thought of getting a "large map of the fortifications of the town and citadel of Namur." He did so, and that was how his hobby got started.
Tristram thinks about certain objections that will be made by the critics, and he answers their charges. He reaffirms that his book is a history. "Of who? what? where? when?" "It is a history-book, Sir, . . . of what passes in a man's own mind." He cites John Locke's Essay Concerning Human Understanding, pointing out that Locke's interpretations do not apply, and he says that Uncle Toby's "life was put in jeopardy by words," not by ideas.
Toby gets his map and studies it. He learns more and more about fortified towns (like Namur) and begins to study all manner of military writings on military architecture, ballistics, trajectories, projectiles. Tristram fears for his uncle's health, and he urges him — as if he were actually there at that moment — to give it up: "Intricate are the troubles which the pursuit of this bewitching phantom, KNOWLEDGE, will bring upon thee. . . . Fly — fly — fly from it as from a serpent. . . . O my uncle! my uncle Toby!"
Tristram tells why he ended the chapter at the "last spirited apostrophe" (it was for the sake of letting it "cool"). Good writers must consider these matters of emphasis and proportion.
Uncle Toby gives up the study of projectiles and turns to the "practical part of fortification only." He begins to long mightily for his recovery, although we don't know yet what he has in mind. Tristram will tell us in the following chapter what Toby has in mind, and after that, "'twill be time to return back to the parlour fire-side, where we left my uncle Toby in the middle of his sentence."
The wound begins to heal nicely, so Toby and his servant, Corporal Trim, embark for Shandy Hall in the country. The reason is that Toby's bedside table was too small to hold all his books and apparatus. When he asked Trim to order him a larger table, Trim suggested that they go to Toby's estate near Shandy Hall; there, under Uncle Toby's expert direction, he would construct on the lawn scale models of the fortifications, complete in every particular so that "it should be worth all the world's riding twenty miles to go and see it." Uncle Toby blushed with joy at the idea, and they are off on his hobby-horse.
Tristram says that the history of their campaigns will make an interesting "underplot in the . . . working up of this drama," but later. "At present the scene must drop, and change for the parlour fire-side."
The author ties together all the facts leading up to and including Uncle Toby's hobby: the setback in his getting well, the first idea for a map of Namur, the military books that fire his imagination, Trim's proposal for the scale models on the bowling green at Shandy Hall, and their setting off to start work. We know all there is to know about that part of Toby's character, and henceforth, when Toby sees everything in relation to that single ruling passion for fortifications, we understand perfectly.
Tristram's references to John Locke again remind us that the author was working in the context of eighteenth-century psychology, philosophy, and epistemology. Even though he cites Locke in order to show a contrary notion, he depends quite a bit on Locke.
The author's continuing close involvement with his characters and his insistence on his job as author is seen in the "spirited apostrophe" to his Uncle Toby and his beginning a fresh chapter. In Chapter 4, he remembers the interrupted sentence — although we may have forgotten it already — and at the end of Chapter 5 he tells us that later he will pick up what is digression (Toby's hobby) and make it a part of the main story. The suspension of the story of Toby's hobby is as calculated as the suspension of Toby's sentence.
Corporal Trim's character is also drawn by means of his hobbyhorse: "The fellow lov'd to advise, — or rather to hear himself talk. . . . Set his tongue a-going, — you had no hold of him." The proposal by Trim to build the models is a perfect demonstration of his oratory; he is involved with Uncle Toby in the fortifications, but that is merely a second-string hobby-horse for him.