But as it is presently defined in the West, Philosophy is the answers to all these questions without God, or without 'revelation'. There is no good reason under why it should be defined this way, but it is: philosophy is defined as finding the great answers of life and death using the human mind, without introducing God or anything supernatural.
So the philosopher sits down without a Bible (or any 'holy book' for that matter) and ponders a subject: war, marriage, and so on, using whatever tools are around her......
...here are five immediate limitations she comes up against:
Limit #1: Experience is a limited guide
One tool she will use is Experience: she will draw on her experience. Trouble is that being finite and located in only one family, one culture, one nation and one short time, her experience will be tiny. Any philosophy based on it will inevitably be open to revision and change in the light of a broader experience.
Limit #2: What 'is' may not be what 'should be'
Another source of philosophy would be the way we find things around us in the world. If you were born into a culture that glorified violence (the Roman world, for example) or tolerated slavery or tolerated abortion, you could easily grow up assuming these "ares" were "rights" (plenty of mothers, yes mothers, fought against the eradication of chimney sweeps, a cruel occupation for a child if ever there was one). If a person feels attracted to someone else of the same sex, one might easily say that this "what is" is "what should be." But who is to say that one can move from 'is' to 'should' that easily? There may be a world of difference between what we find around us in the world and what is actually right: 'is' is no necessary guide to 'should be'. What if something has gone wrong with the world which means what 'is' is a distortion of what 'should be'? But, without revelation, how would a philosopher know that what 'is' is not right?
Limit #3: The experts may be wrong
Suppose we go to the experts to form our philosophy. They could also so easily be wrong. New textbooks are written every year for exactly that reason. What the experts say in one decade is overturned in the next. Sometimes, as in the case of Marx and Chairman Mao, not before considerable carnage has been done to and by the followers of these false systems.
Limit #4: The balance of truth
What if in our age or particular culture (or just in us personally!) there is a really big imbalance in perspective? In a recent book called "The Master and his Emissary" the author Iain Mcgilchrist suggests that present Western culture, unlike Eastern cultures, is dominated by left-hemisphere brain function, with potentially dire consequences for the future. What if there are other imbalances which we simply do not see, all of them skewing our philosophy away from the truth?
Limit #5: There may be things unknowable by human research
Suppose there are keys or key pieces of information that simply cannot be known without 'revelation'? Suppose there are secrets which only the Creator can reveal to us, information which simply can't be found by us, no matter how big the research programme? Any philosophy which ignores them would then turn out to be faulty.
For all these reasons philosophy has severe limitations.
The overarching reason all human-without-God philosophy is flawed is this: the people who create it are tiny and finite and in the Great Scheme of Things, and simply don't know that much at all: none of us do, that's why we need revelation.
Philosophy is like a bridge that gets us partway to the land of truth. But we need revelation to get over to the other side. If we walk over the bridge thinking it will lead us to truth, we'll end up stumbling off the unfinishable deck into the river of error.