Epistemology is the name given to the theory of knowledge. How do we know anything, and how certain is our knowledge? The topic is a massive one: much debated by philosophers from the time of Plato onward. I do not claim to be a philosopher, and what I'm offering here, with some trepidation, is just a few thoughts of my own on the subject.
Classical British philosophers of the 17th to 19th centuries, from Bacon and Hobbes through to John Stuart Mill, generally saw knowledge as coming through empiricism (a word derived from a Roman Epicurean philosopher, Sextus Empiricus). This derives our knowledge of the material world through the experience of our senses. Knowledge is established through perception, and is refined by observation and experiment. This carries the presumption that only the material world is real, and what cannot be observed by the senses (metaphysics) is unreal, even non-existent.
It has been said that philosophers tend to start by talking about tables and chairs, probably because these are present in the rooms where they hold their discussions. So if we take a table, what can we know about it?
Our senses tell us that it has a certain shape,dimensions, colour and texture. If we hit it, a certain noise results. It has a smell and a taste as well, though these are less important to us. (A dog, by contrast, would probably describe the table in terms of its smell) Using other knowledge, I can deduce that my table is a man-made object, constructed from wood. An intelligent person who had never seen a table (a paleolithic human, or an alien life-form?) might guess, from its flat, level surface, that it was constructed for the purpose of having things placed upon it.
But how far can we trust our senses? Descartes based his philosophical system on his famous "Cogito ergo sum". He thought: what can I know for sure? Well, here I am, sitting at my desk - or at least, I think I am: but many times in the past I thought I was, only to awake and realize I'd been dreaming. Or I might be mad, or bewitched, or I might even have died and failed to notice it. But the one thing I can be sure of is that, even if I'm asleep, dead or mad, I'm undeniably thinking about the problem. Hence, "Cogito ergo sum": "I think, therefore I must exist". (Some philosophers have denied the validity of this, on the grounds that all he had proved was that there was some thinking taking place somewhere)
A point occurs to me here. As mentioned above, my senses perceive certain things about my table. But I might be dreaming about the table, in which case there was nothing there in reality. If I was drunk, I might perceive the table as moving about. If I had taken LSD, I might perceive the table as a purple elephant. If I was a paranoid schizophrenic, I might think the table was persecuting me. So what justification do I have in thinking that the table I perceive when I am sane, sober and awake is the "real" table, and the table I perceive when I am drunk, drugged, mad or fast asleep is "unreal"? If all are equally valid, then I would say that scientific knowledge is impossible. Or is this just a cultural prejudice? Hippies in the 1960s thought that perceptions obtained under the influence of drugs were perfectly valid, and the validity of dream experiences is given support in the Bible. Some philosophers thought this conundrum could only be resolved by bringing God into the occasion: the table as perceived when I am sane, sober and awake is the table as perceived by God, and the other perceptions are thus unreal and false. This was sometimes seen as a strong argument for the existence of God, without which there could only be chaos.
The question of whether we have any sources of knowledge other than empirical investigation is a vast one, which I shall leave for a later occasion. Instead I would like to pass on to a different issue: why do I call the object I perceive a "table"?The most obvious answer is "Because it looks like a table", which takes us straight to Plato and his notion of the "forms" or "ideas". We are aware of a "form" of an ideal table, to which objects may bear a close enough resemblance to be classed as "tables". Furthermore, the actual table I have before me now is only a temporary object: it is only a few years old, and its lifespan as a table will probably be quite short; but the table "form" is eternal. Mystical Neo-Platonism might speculate that the "forms" exist somewhere: in the mind of God, perhaps? (Plato's theories seem most applicable to mathematics. Two plus two would make four and Pythagoras's theorem would still be true even if there was no intelligent life anywhere in the universe)
Can something act as a table without being a table? Clearly, yes: if I sit on a table it won't have become a chair; neither will my chair become a table if I rest a cup of tea on the seat. Students once asked me the following: if Pedley, one of their number, went down on all fours and let someone put a plate of food on his back, would he have become a table? We thought not. But what if, after he died, Pedley was stuffed and mounted in this position and used for this purpose: would he have become a table then? We thought in that case, probably yes.