In Thomson's Castle of Indolence, to which Armstrong contributed four stanzas, describing the diseases incidental to sloth, he is depicted as the shy and splenetic personage, who, "quite detested talk." His portrait is drawn in Thomson's happiest manner:
With him was sometimes joined in silent walk
(Profoundly silent, for they never spoke),
One shyer still, who quite detested talk;
Oft stung by spleen, at once away he broke,
To groves of pine and broad o'ershadowing oak;
There inly thrilled, he wandered all alone,
And on himself his pensive fury wroke;
Nor ever uttered word, save, when first shone,
The glittering star of eve — "Thank heaven, the day is done!"
The poet was of a somewhat querulous temper, and his friend Thomson remarked of him, "The doctor does not decrease in spleen; but there is a certain kind of spleen that is both humane and agreeable, like that in the play."