If thoughts depend on memories and memories are unreliable then how can we trust any thought? I assume thoughts require memories because thoughts seem to require at least some time to compute, even with very simple thoughts we think thing one at a time - if it's not quite like that I think it's very close to something like that, maybe my whole doubt depends on a dubious connection between thought and memory, I don't know. I think the unreliability of memory is more obvious, memory seems to be something just given to us and we simply have to "trust" it but the possibility of doubt is still there. I recognize that there is some not inconsiderable paradox in doubting the very idea of being able to form a thought and using thought to achieve that doubt but alas... I wonder if this suggests that thought in its truest form is something more intuitive and directly related to a grasp of the present moment than reason as it is generally understand as a discursive process.

Thanks for your question. I'd distinguish the undeniable claim that memory is fallible from the less plausible claim that memory is unreliable. I'm no psychologist, but it seems that the reliability of memory comes in degrees, depending on who's using it, under what conditions, and what its content is. The kind of remembering described in your question -- remembering what I was thinking just an instant ago -- doesn't seem especially unreliable, under favorable conditions anyway. Furthermore, we logically presuppose the reliability of memory in general even as we check whether some particular memory of ours is false: We ask those who are better-positioned what they remember, we trust that we correctly remember the meanings of words they use in their answers or the meanings of words we read in contemporaneous accounts of the event, and so on. Indeed, if we persist for any length of time in our belief that memory is fallible, that too depends on trusting our memory: it presupposes that we correctly remember that memory led us astray at least once in the past. So there seem to be limits to how sweeping any rational doubts about memory can become.

"I recognize that there is some not inconsiderable paradox in doubting the very idea of being able to form a thought and using thought to achieve that doubt". Well spotted! Suppose that your doubts about memory lead you this: "I cannot trust any thought, including this one". Where do you go from there? It doesn't look as though the paradoxical nature the thought undermines it in such a way that you can conclude that it is false, and proceed to trust some thoughts. It sort of leaves you with nowhere to go.

I agree with Stephen. Memory is not that unreliable. It is much less reliable than we think. When we seem to remember things our brains seem to do a lot of construction and interpretation, and present to us a partly made-up image of some past even as if it were a perfectly accurate representation. This can get us into trouble. But our short-term memory is pretty good and serves its purpose. It is not hard to keep track of the thoughts involved in a short line of reasoning. It also gets a lot easier if we write them down. We can the create longer lines of reasoning by understanding shorter ones and stringing their conclusions together, keeping track of the overall structure. Memory, combined with pen and paper (or today's equivalents) is good enough to support reason as a discursive process.

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