(From The Wrecker by Robert Louis Stevenson and Lloyd Osbourne, Chapter VI, published 1892)
Passage 96
Pinkerton was in the waiting-room, feverishly jotting in his pocket-book. As he saw me enter, he sprang up, and I declare the tears were trickling on his cheeks.
"My dear boy," he cried, "I can never forgive myself, and you can never forgive me. Never mind: I did it for the best. And how nobly you clung on! I dreaded we should have had to return the money at the doors."
"It would have been more honest if we had," said I.
The pressmen followed me, Harry Miller in the front ranks; and I was amazed to find them, on the whole, a pleasant set of lads, probably more sinned against than sinning, and even Harry Miller apparently a gentleman. I had in oysters and champagne—for the receipts were excellent—and being in a high state of nervous tension, kept the table in a roar. Indeed, I was never in my life so well inspired as when I described my vigil over Harry Miller's literature or the series of my emotions as I faced the audience. The lads vowed I was the soul of good company and the prince of lecturers; and—so wonderful an institution is the popular press—if you had seen the notices next day in all the papers, you must have supposed my evening's entertainment an unqualified success.
. . .
Does the sentence if you had seen the notices next day in all the papers, you must have supposed my evening's entertainment an unqualified success express reality or unreality? It is a conditional sentence for a past situation, but in must have supposed the modal verb must isn't 'shifted' or 'moved back' in time like will -> would (have supposed). Now, what does this sentence express - reality or unreality? Is it true what the sentence says or is it just imagined?
Edit:
To add more details: In this sentence there is the modal verb must. The modal verb must doesn't refer to the past and must isn't a distancing form; or, to put it differently, must doesn't have forms like would (used as a distancing form of will) or like might (used as a distancing form of may). However, when a speaker wants to state an unreality or a merely imagined action or occurrence, when a speaker states a counterfactual conditional sentence it (s/he ?) needs a distancing form of the modal verb in the main clause as in the sentence
If you had not passed the test, you would have failed the course.
In our sentence . . .
If you had seen the notices next day in all the papers, you must have supposed my evening's entertainment an unqualified success
. . . there isn't any modal verb form equivalent to would (must doesn't have one; we use would have to instead). That's why I suppose our sentence is expressing a reality: The speaker is stating that the result is a consequence of some really possible prior action or state (in this case future in the past of course). We also use must + have + ...ed form to talk about deductions and conclusions in the past. Even though there is the verb suppose in the main clause 'you must have supposed my evening's entertainment an unqualified success', the speaker is stating that supposing his success is a reality and not merely imagined, the speaker asserts or claims that you must have supposed this. That is to say the main clause, hence the whole conditional sentence is talking about a real and not merely imagined deduction or conclusion, about a fact/factum in the past depending on whether you had seen the notices in the papers or not. Am I right or am I wrong? That's my question.
