You appeared to be surprised when I told you, on our first meeting, that you had come from Afghanistan." "You were told, no doubt." "Nothing of the sort.

I _knew_ you came from Afghanistan.

From long habit the train of thoughts ran so swiftly through my mind, that I arrived at the conclusion without being conscious of intermediate steps.
There were such steps, however.

The train of reasoning ran, 'Here is a gentleman of a medical type, but with the air of a military man.

Clearly an army doctor, then.