Knowledge is a relationship;
certainty is a feeling.
Knowledge is a name for the correspondence between a thinking mind and an actual world. Certainty is a name for the conviction within the being of the thinking mind of the exactitude or beauty of the relationship between mind and world. To know with cartesian certainty, as they call it in honor of the solitary frenchman, is to be certain of knowledge's correctness (compliance of mind-image with world-existence) with perfect justification. But what does it mean to be justified. Is it a reality of perception or of actual existence? I mean, to be justified must a conscious being say we are so (either ourselves or others) or can we be so by existence. If knowledge is a relationship, than justification would mean "right-placement" - or the correct position of mind to world. It would be discussing an actual relationship between objects. But I think that in the age old "justified true belief" definition it is being said that it is a perceptual/subjective occurance in the mind of a being - you aren't justified till I or you feel that you are, till we feel that in order to prove the rightness of your knowledge, your mind's relationship to real world (and to other real minds), you have used every reasonable proof and made every abstract and physical experiment to demonstrate that it could be no other way. Of course this is folly - it would never happen - never can. Therefore: "I think" is the only justified true belief. And it is only justified to yourself.
We do not live in a world of certainty - we live in a world of time and space. Knowledge and belief are completely different kinds of creatures. Knowledge describes the mirroring of a world inside a mind, and belief describes a feeling of rightness of a certain kind of feelings about perception of knowledge. Belief is a complicated occurance. And yet here we are, living by it every moment.
Science is a name for the tool by which knowledge can be modified by the sensing bodies connected to our minds - the sensual communication of the thinking mind's body to the world, and lingual communication between minds about this sensual communication with the world. Because we believe in God (who would like to try and restate this?) we beleive that the world is actually present outside of our minds, that knowledge is not an illusion, and that we may build, through language, correct knowledges about the world throughout many lifespans. Students using science say "look, I can touch and see the world outside of my mind using my body - want me to prove it, that my perceptions are real? See, watch."
We watch and believe our perceptions of his sensual communication with a real world. That is, we have feelings that our intellections of his actual presence and communication with a real world are justified - or correctly reflecting actual object to object relationship.
Knowledge is a relationship between our minds and a real world. True knowledge is a correct realtionship between our minds and a real world. Certainty is a feeling we have about the correctness our knowledge.
(intellectualization of our perceptions of a real world)