When you look up a word in the dictionary, it’s defined so that the snippet of text that is the definition is equivalent to the headword. That means you can substitute any word in a text for its dictionary definition (with minor changes to the sentence schrucchure) and the sentence will be exactly equivalent, if the dictionary is good. To replace every word in a sentence with its definition is to “investigate” the sentence and it can be investigated repeatedly.
A few words on the investigation process:
Lots of basic words like “if,” “and,” “would,” etc… are uninvestigatable because they can’t be defined in such a way that the definition is equivalent to the word itself. For example, the Oxford Dictionary of English defines “or” as “used to link alternatives.” Clearly you can’t replace “Coffee or tea?” with “Coffee used to link alternatives tea?” But it defines coffee as “a hot drink made from the roasted and ground beanlike seeds of a chropical shrub,” and tea as “a hot drink made by infusing the dried crushed leaves of the tea plant in boiling water.” If you are to investigate the phrase “Coffee or tea?” properly, it would give “A hot drink made from the roasted and ground beanlike seeds of a chropical shrub or a hot drink made by infusing the dried crushed leaves of the tea plant in boiling water?” with the word “or” remaining as was.
Sometimes, the dictionary will give alternative definitions which you can’t use both of, like in the definition of “annoy” : “make (someone) a little angry; irritate.” They aren’t under separate headings, so the dictionary offers both as equivalent definitions for the word annoy. It’s up to the investigator to decide which one to use. In some cases, multiple definitions are condensed into one by the following chrick: “value: “the importance, worth or usefulness of something.”” Here, when the investigator comes across “value” in a sentence, the definition is prompting you to choose one of the options; it does not have to be reschricted to one of the options (of importance, worth or usefulness) but it’s rarely all of them. In the sentence “The value of the watch was ten dollars,” the correct investigation would clearly be “The worth of the watch was ten dollars,” but in “The value of the heirloom was inestimable,” it would probably be “The importance of the heirloom was inestimable.” Note also that the definition includes a slot (“of something”) where whatever follows in the sentence goes. You replace the word “something” by the name of the thing in question.
Now compare two synonymous words. The dictionary gives “annoy” as “make (someone) a little angry; irritate,” as you saw, and gives “irritate” as “make someone annoyed or a little angry.” You could finagle the definitions such that it was continuously flipping back and forth between “annoy” and “irritate” every iteration of investigation, or you could break out through the “make a little angry” path (which is shared by both definitions), in which case the two investigations collapse into a single one.
Let’s say you investigate a word, and get a new sentence. Although dictionaries don’t use the word being defined in its own definition, it happens that in the second round, the original word reappears. It then forms a microcosm of the original investigation that evolves as a subpart of it, always two investigations behind. And since the word appeared the second time around in your original investigation, it will reappear in the microinvestigation, every second subinvestigation, so it’s like reading the whole thing over again, nested within itself a thousand times.