Nietzsche accuses past philosophers of lacking critical sense.

Nietzsche wastes no time by starting with some strong words against the ancient philosophers and religions. With a very logical, although complex argument, Nietzsche suggests that all religion and philosophy can be summarized into simple prejudice; a prejudice that--not coincidentally--supports the perspective of the philosopher or preacher. Nietzsche claims that because of their limitations, religion and philosophy cannot uncover universal truths.

On the other hand, Nietzsche views himself, and acknowledges that there may be others, that are free spirits. A free spirit is someone who is truly able to think for themselves and not be biased by society, the outcomes of their truths, or personal incentive, but only by the mission to identify things that are false. Nietzsche believed that an absolute truth as difficult to find, but finding falsehoods was easier and could lead to truth.

One of the concepts Nietzsche is most well-known for, much of which comes out of Beyond Good and Evil, is that truth and thoughts are ever-changing and developing, which is why the ideas embraced by religions and philosophers in one age may not be considered 'truth' at some later time. While 'the world is flat' is a cliché example, it's exactly the process Nietzsche expects all thoughts and ideas to follow.

The drivers of changing truths and facts are power and will, according to Nietzsche. So those that hold power--or the power inherent in strongly held, popular thoughts--may define truth today, but when that power base shifts, so might the definition of morality, good, and evil - hence the name of the book: Beyond Good and Evil.