Monica often puzzled him gravely; he could not find the key to her satisfactions and discontents.

To regard her simply as a human being was beyond the reach of his intelligence.

He cast the blame of his difficulties upon sex, and paid more attention to the hints on such afforded him by his reading.

He would endeavour to keep his jealousy out of sight, lest the mysterious tendency of the female nature might prompt Monica to deliberate wrongdoing.
To-day for the first time there flashed across him the thought that already he might have been deceived.

It originated in a peculiarity of Monica's behaviour at luncheon.