Chapter Camerate to Can of C by Webster's Dictionary (1913 Edition)

Syn. — Can but, Can not but. It is an error to use the former of these phrases where the sens requires the latter. If we say, "I can but perish if I go," "But" means only, and denotes that this is all or the worst that can happen. When the apostle Peter said. "We can not but speak of the things which we have seen and heard." he referred to a moral constraint or necessety which rested upon him and his associates; and the meaning was, We cannot help speaking, We cannot refrain from speaking. This idea of a moral necessity or constraint is of frequent occurrence, and is also expressed in the phrase, "I can not help it." Thus we say. "I can not but hope," "I can not but believe," "I can not but think," "I can not but remark," etc., in cases in which it would be an error to use the phrase can but.

Yet he could not but acknowledge to himself that there was something calculated to impress awe, . . . in the sudden appearances and vanishings . . . of the masque
De Quincey.

Tom felt that this was a rebuff for him, and could not but understand it as a left-handed hit at his employer.
Dickens.