During this period, 1639--1649, while his interests were engaged by the all-absorbing events of the civil strife, he wrote no poetry, or none deserving the name. All artists have intervals of non-productiveness, usually caused by exhaustion. This was not Milton's case. His genius was not his master, nor could it pass, like that of Leonardo da Vinci, unmoved through the most tragic scenes. He deliberately suspended it at the call of what he believed to be duty to his country. His unrivalled power of expression was placed at the service of a passionate political conviction. This prostitution of faculty avenged itself; for when he did turn to poetry, his strength was gone from him. The period is chiefly marked, by sonnets, not many, one in a year, or thereabouts. That _On the religious memory of Mrs. Catherine Thomson_, in 1646, is the lowest point touched by Milton in poetry, for his metrical psalms do not deserve the name. The sonnet, or Elegy on Mrs. Catherine Thomson in the form of a sonnet, though in poetical merit not distinguishable from the average religious verse of the Caroline age, has an interest for the biographer. It breathes a holy calm that is in sharp contrast with the angry virulence of the pamphlets, which were being written at this very time by the same pen. Amid his intemperate denunciations of his political and ecclesiastical foes, it seems that Milton did not inwardly forfeit the peace which passeth all understanding. He had formerly said himself (_Doctrine and Disc._), "nothing more than disturbance of mind suspends us from approaching to God." Now, out of all the clamour and the bitterness of the battle of the sects, he can retire and be alone with his heavenly aspirations, which have lost none of their ardour by having laid aside all their sectarianism. His genius has forsaken him, but his soul still glows with the fervour of devotion. And even of this sonnet we may say what Ellis says of Catullus, that Milton never ceases to be a poet, even when his words are most prosaic.