Johnson here means to express in another form what Dryden says of himself:
'Nor wine nor love could ever see me gay,
To writing bred, I knew not what to say.'(Satire to his Muse.)
But though this is clearly the meaning Johnson intends to convey, it is not so clear what is the precise grammatical construction of the words. The sentence may be understood to convey that the intellectual treasures were locked up away from his own use, out of his own reach, so that he could not readily get at them for the purposes of conversation. The next sentence then presents a contrast to this, mentioning the extreme facility with which he wrote. There is, however, another construction possible, which would express the same meaning in a somewhat different form. Use may here mean habit or custom; and we may construe the sentence,
'The treasures of his intellect were great, but, from his habit (of mind and life, in writing so much), locked up.'
The next sentence would then be corroborative of this, as tending to show how completely Dryden had accustomed himself to express his thoughts only in writing. Though the actual meaning of the whole passage cannot he in doubt, either construction seems somewhat strained, and the student must decide between them for himself. However it be construed, the sentence must remain one of those rare cases where the carelessness which Johnson often displays about his matter has crept into his style.