Kyla asked:

Is objective knowledge possible? How would some famous philosophers answer this question?

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Knowledge is often defined as 'justified true belief'. We have to believe that our claim is true, it has to
be actually true, and we have to be justified in believing that it is true.

Rationalists and empiricists disagree over the source of knowledge or what counts as relevant
evidence. Skeptics suppose that the justification condition is never met.

In the last century, Gettier questioned whether all three are truly sufficient for knowledge. A.C.
Grayling has set out a definition of skepticism presumed in most contemporary discussions:

Possibly [Not P and E]

where P is some proposition and E the best evidence in support of it. The skeptic says that even the
best evidence we have in support of P is logically consistent with a denial of P. So how can we know
anything?

Empiricists such as Locke, Berkeley and Hume all thought that knowledge stems from experience; it
appears obvious that we use our senses to take in the information that we use to form concepts.
Perceptions give thought its content. Yet this does not pass the skeptical test above: how, on the
basis of experience alone, do I make the leap from the way (say) light bashes on my retina or the way
sound waves bash on my inner ear to the claim that I know the world looks and hears thus and so?

Perhaps the truths of reason are what we know. Descartes and Leibniz thought so. We cannot doubt
that 2 + 2 = 4. If this is the case then it is doubly clear that there is a problem with pure empiricism
since what sensory experience could be as indubitable as a mathematical axiom? Descartes
suggested that we might not know we are right even about logical truths. One can't imagine them
being wrong, but is this an adequate justification for thinking we have objective knowledge of them?
We might be being deceived. His way out was to prove the existence of God who wouldn't deceive
because he is loving. It is not clear that this is the best strategy.

If you think all this a bit depressing, responding to skepticism is not impossible once you start to think
about what needs to be the casein order for there to be knowledge. What needs to be true of mind
and world in order to make sense of the claim that I know something?

This is Kant's strategy. That there is no such thing as perspectiveless knowledge — that is, in order
for something to be known it must be known from some perspective or other; so there is a sense in
which objectivity must conform to a way of seeing things. What is known must conform to the
perspective of the knower. What comes out of this is that we need the senses to input information, but
we also require the mind to bring certain 'forms' to raw sense-data in order to make sense of
experience. We require forms of thought which we couldn't learn from experience because they are
presupposed in it. 'Percepts without concepts are blind' as Kant puts it.

In the mainstream tradition of European philosophy is the thought that a person 'knows' how things
are with him (that he is in pain for example) but has a problem inferring to how things are on the
outside. The private world is known better than the public. Descartes et al took this as given. It makes
no sense to doubt whether we are in pain, so we assume it is something we know. It was natural to
assume that this is a paradigm case of complete certainty. The problem here is that if we obtain all
our concepts of the mental through introspection alone, I would need a good argument for reasoning
from my own case that this goes on in others.

This approach is misleading; the ability to state one's pain does not rest on evidence — I do not
discover or check that I am in pain. If being ignorant about one's mental states makes no sense then
neither does knowing about them. Peter Hacker puts it this way:

To say 'I know I am in pain' is either an emphatic avowal of pain or a philosophers nonsense.

A possible way out then is to follow a Kantian type of strategy for dealing with skepticism together
with this thought from Peter Strawson (IndividualsCh. 3 'Persons'): it makes no sense to ascribe
consciousness to oneself unless we already know how to ascribe some states of consciousness to
others. This is what close to what Wittgenstein says in the Philosophical Investigations: we need to
master a language before we can think or say anything, and we can master language only within a
shared 'form of life', in a community of other beings. So a possible response to skepticism is to say
there are certain things about the world and the mind which need to be in place before we can even
get started talking about what we know, or what it makes sense to say that we know. Needless to say
a lot of work would need to be done to make this approach sufficiently robust; but that is when you
start to realise how flexible the idea of "knowledge" really is.

Adam Gatward