One mistake we sometimes make is to confuse a memory of a decision to do something with a memory of having done it. "I thought I did that! No, my man, you merely thought of doing it."
One morning I wasted time searching for an article I had printed out the day before. But I was searching for a nonexistent object. I hadn't printed it out; I had merely resolved to do so. I confused resolve with result.
Memory, then, is fallible. But it is via memory that we know this. The non-veridical memory of having printed the document is known to be non-veridical by comparison with the veridical memory of having intended to print it. So while fallible, memory is a source of knowledge, and is generally reliable, although its powers vary from person to person.
Veridical memory of wholly past events gives the lie to presentism, the view that the present alone exists. For if the present alone exists, then the wholly past does not exist. But what does not exist cannot be known. Given that some memories are veridical, and give us knowledge of wholly past events, events that could not be known f they did not exist, presentism is false. (To follow this argument you must avoid the mistake of thinking that ‘exists’ can only mean ‘exists now.’)
"So what are you saying, man? That the past is real?"
That’s what I am saying, son. The wholly past, though past, is actual and factual: not merely possible, not potential, and not fictional. Deny that, and you have divested historians of their subject-matter. I could go on, but it’s Saturday night, and time for a drink, an event which, though not yet actual, will soon become actual, and forever after remain actual. Having said that, I must go on a bit longer.
Socrates’ drinking of the hemlock, though a logically contingent event, acquired after its occurrence the curious sempiternal status of necessitas per accidens, accidental necessity. After its occurrence, the event became such that nothing and no one, not even God, could undo or ‘cancel’ it. As Thomas Aquinas says somewhere, not even God can restore a virgin. The same goes for my upcoming Negroni: once drunk, its having been drunk can never be erased from the roster of actuality.