There are infinite possible assertions I can make, positive and negative. I am capable of making many of these possible assertions into beliefs, but there are infinite possible assertions I will never even consider. I am trying to discover as many true assertions as I can so I can believe them, in other words, I am trying to believe those I know are true.
Let's take a simple example, what is in the room with me. There are infinite possible assertions. "There is a stapler in here." "There is not a word in here." "There are quarks in here." "There is a dinosaur in here." ad infinitum. To immediately take a stance on any of these is to already be in the grips of a metaphysical theory and interpret the words in definite senses to the exclusion of others. This is starting from unsupported presuppositions and so can never be rational. I don't know reality, I don't know the various realms of being, that is what I'm trying to figure out. For all I know there can be senses in which there is a dinosaur in the room with me, overlapping frames of time, multiple dimensions undetectable to my puny human senses, etc. However, the point is that I can go through an infinite list of possible monsters and goblins.