assert positively

References in classic literature ?
"I assert positively that I lived on friendly terms with my wife, allowing, of course, for the little occasional disagreements and misunderstandings of married life.
There is no reason why memory-images, accompanied by that very simple belief-feeling which we decided to be the essence of memory, should not have occurred before language arose; indeed, it would be rash to assert positively that memory of this sort does not occur among the higher animals.
Hence, correlationism must assert positively one absolute fact: the facticity of the contingent relation it postulates.