Assent on reasonings not demonstrative is too widely recognized an act to be irrational, unless man's nature is irrational, too familiar to the prudent and clear-minded to be an infirmity or an extravagance. None of us can think or act without the acceptance of truths, not intuitive, not demonstrated, yet sovereign. If our nature has any constitution, any laws, one of them is this absolute reception of propositions as true, which lie outside the narrow range of conclusions to which logic, formal or virtual, is tethered; nor has any philosophical theory the power to force on us a rule which will not work for a day.
John Henry Newman, An Essay in Aid of a Grammar of Assent, Chapter 6. In the sense Newman means, 'assent' is unconditional and does not admit of degrees; if you assent to something, you simply take it to be true, period. (You can still have a reason for assenting, of course; but the assent itself is not provisional or tentative or partial or something that comes in any kind of degree at all.) Examples of things that Newman thinks all reasonable people, however much they love truth, take to be simply true, despite not having perfect proof of it, are: that we exist, that there are things we do not know, that there are other minds, that there is an external world, that the earth is round, that there are lots of cities on it and that they do not disappear when we leave them, that we have parents, that we will die. The idea is that whether or not one can have a rigorous proof of these conclusions, in fact every reasonable person accepts some things like these as simply true despite having nothing more than a probable -- and sometimes even only a weakly probable -- argument in hand for them; therefore it can be reasonable to believe something certainly on reasoning that itself is only probable.