not certain

References in classic literature ?
My friend has an excellent understanding-- though I am not certain that I consider her marrying Mr.
They had not met for two or three weeks, and Mary had much to say to Ralph; but she was not certain how far he wished for her company.
Becky supposed she would be glad of that, and she tried to believe she was glad of it, but she found she was not certain.
But it is not only that which drives me to despair; for what I am not certain of, madame, is that the man whom Christine loves is worthy of her love
I did not know that; I am not certain that I know it now.
She was not certain where he lived, but it was in one of the three houses opposite.
However, it is not certain that Halpin Frayser came to his death by exposure.
The reader may suppose Mrs Miller received this account with great thankfulness, and no less pleasure; but so uncommon was her friendship to Jones, that I am not certain whether the uneasiness she suffered for his sake did not overbalance her satisfaction at hearing a piece of news tending so much to the happiness of her own family; nor whether even this very news, as it reminded her of the obligations she had to Jones, did not hurt as well as please her; when her grateful heart said to her, "While my own family is happy, how miserable is the poor creature to whose generosity we owe the beginning of all this happiness
Still it is thought, though it is not certain, that we occasionally see the actual surface of this orb, an advantage we do not possess as respects any other of the heavenly bodies, with the exception of the moon and Mars.
He turned up the bank, not without some fear lest he might miss the right way, since he was not certain whether the light were in front or on the side of the cottage.
I am not certain of that,' rejoined Doyce, laying his hand persuasively on his shoulder.
We discussed everything we knew, during the first fifteen or twenty minutes, that morning, and then branched out into the glad, free, boundless realm of the things we were not certain about.