Why is man presumed to be good?

Only the misguided or shortsighted might have such a monopole vision, and historically there have been those of the philosophical persuasion who argued it, especially during the late 16th to 19th Centuries Two famous English Philosophers who taught who perceived man to be basically good or basically bad were john Locke and Thomas Hobbs - Lockean Consensus v. Hobbsian Consensus.

In most philosophies, man is not seen as only good or only evil; rather, he is a complex combination of both.

The Bible teaches man has the ability to use both the knowledge of the tree of good and evil. Other schools of thought and wisdom teach likewise. A small sample of these is below:

Romans 7:21-23New International Version (NIV)

"So I find this law at work: Although I want to do good, evil is right there with me. / For in my inner being I delight in God's law; / but I see another law at work in me, waging war against the law of my mind and making me a prisoner of the law of sin at work within me."

William Shakespeare, Julius Caesar, Mark Antony's speach, Act II Scene iii:"The evil that men do lives after them. / The good is oft interred with their bones." Albert Einstein"The world is a dangerous place to live; not because of the people who are evil, but because of the people who don't do anything about it." Siddhārtha Gautama (The Buddha)"It is a man's own mind, not his enemy or foe, that lures him to evil ways." Chief Joseph, Nez Perce"We were taught to believe that the Great Spirit sees and hears everything, and that he never forgets, that hereafter he will give every man a spirit home according to his deserts; If he has been a good man, he will have a good home; if he has been a bad man, he will have a bad home. This I believe, and all my people believe the same." Letakos-Lesa, Eagle Chief of the Pawnee"All things in the world are two. In our minds we are two, good and evil. With our eyes we see two things, things that are fair and things that are ugly.... We have the right hand that strikes and makes for evil, and we have the left hand full of kindness, near the heart. One foot may lead us to an evil way, the other foot may lead us to a good. So are all things two, all two."