I have reduced the superficial muscular phenomena and facial symptomatic aspect of such people to an exact science founded upon a schedule approximating the Bertillon system of records.

And," he added, smiling, "out of the twenty-seven known vocal variations your voice betrays twenty-five unmistakable symptoms; and out of the sixteen reflex muscular symptoms your face has furnished six, your hands three, your limbs and feet six.
Then there are other superficial symptoms--" "Good heavens!" broke in Harren; "how can you prove a man to be in love when he himself doesn't know whether he is or not?
If a man isn't in love no Bertillon system can make him so; and if a man doesn't know whether or not he is in love, who can tell him the truth ?" "I can," said the Tracer calmly.
"What! When I tell you I myself don't know ?" "_That_," said the Tracer, smiling, "is the final and convincing symptom.

_You_ don't know.

_I_ know because you _don't_ know.

That is the easiest way to be sure that you are in love, Captain Harren, because you always are when you are not sure.