He ... told Antonius plainly, that his fortune (which of it selfe was excellent good, and very great) was altogether bleamished, and obscured by Caesars fortune: and therefore he counselled him utterly to leave his company, and to get him as farre from him as he could. “For thy Demon,” said he (that is to say, the good angell and spirit that kepeth thee), “is affraied of his, and being coragious and high when he is alone, becometh fearefull and timerous when he commeth neere unto the other.”
Shakespeare was to make use of this in detail when he drew on the Life for an independent play.
O Antony, stay not by his side:
Thy demon, that’s thy spirit which keeps thee, is
Noble, courageous, high, unmatchable
Where Caesar’s is not; but, near him, thy angel
Becomes a fear, as being o’erpower’d: therefore
Make space enough between you.
(II. iii. 18.)
But already in Macbeth it suggests a simile, when the King gives words to his mistrust of Banquo: