A Debate on the Nature of Belief
The medieval philosopher Anselm of Canterbury (1033-1109 AD) argued that faith in something was evidence for the existence of the thing believed in. Anselm was a scholastic; therefore, he did not worry about whether or not the argument was logical so much as it supported established wisdom (Church doctrine).. In particular, the Apostle Paul remarked in Hebrews 11: 1 that faith was "evidence in things not seen."

There is a simple problem with Anselm's logic: simply believing something exists does not make that thing exist any more than disbelieving in the existence of a thing makes that thing cease to exist. Yet, Anselm was a scholastic philosopher; therefore, he explained away this apparent contradiction by constructing the following supporting argument (which came to be known as the Ontological Argument): we would not even have an idea or concept of god in our mind if this being did not already first exist.

williamOckham's Reponse
A century and a half after Anselm first proposed his argument, the monk and philosopher William of Ockham (1287-1347 AD) refuted Anselm's so-called Ontological Argument.

First off, it should be noted William of Ockham wasn't an atheist any more than Vesalius was; it just so happens that the work of both men placed in to question long held belefs that were deeply felt. In reality, both men did a great service to the advancement of human knowledge by helping improve the quality of our understanding.

For his part William appreciated Thales of Miletus' (624-546 BCE) approach to learning about the world. Thales was an ancient Greek philosopher. Unlike Anselm, Thales did not just use arguments to prove or disprove something; instead, Thales actually conducted experiments to test whether his ideas had any merit; moreover, there was a tendency in Thales' time to invoke the work of the gods to explain mysteries, e.g. there was a lightning storm therefore the gods were displeased. Thales did not invoke the gods. Instead he looked for natural and logically consistent explanations for why things were the way they were, e.g. the lightning storm was a natural process which had nothing to do with the attitude of a god.

When it came to disproving Anselm's Ontological Argument, William of Ockham could only use logic. In the great scheme of things, the existence of God simply is not falsifiable. William therefore addressed whether or not Anselm's argument was logically supportable by testing its plausibility.

William put forth the following argument, i.e. if faith was evidence for the existence of God then faith in the existence of unicorns would likewise be proof of the existence of a one-horned white horse. Ockham pointed out that while faith in God appeared to be proof for God's existence, Anselm's argument did not stand up to testing when applied to unicorns.

Therefore, to be consistent William rejected the idea that belief in God was evidence for anything; he also asserted with equal vigor that believing God did not exist did not cause God to cease to exist. In the end, God did not depend upon a person's belief to exist any more than you or I depend upon someone believing in our existence; further still, God could not be postulated out of existence by simply disbelieving in Him. His existence was entirely independent of a person's belief or lack thereof. Logically, this led William to the following conclusion: God, you, your friend, that cat over there, even that chair, etc. all exist despite of our belief and not because of it.

Yet, for William to successfully refute Anselm he had to tackle Anselm's supporting idea, e.g. How would the idea or concept of god enter the human mind if "god" did not already exist in reality?

William used the following two arguments to place the ontological argument in to doubt: firstly, there is a difference between a concept and an actual thing. For example, an actual thing like a white horse or a single-horned goat do exist in reality.

This is an indisputable fact.

But there is no such thing as a horse, as defined, that has a horn. Let's look at another example: while short Irishmen most certainly exist, it is doubtful that if you chased one down they'd necessarily give you a pot of gold (as per a leprechaun). Yet, the concept of a leprechaun exists. But this concept is just a variation on something that does exist. The reason the concept appears to be as true as the real thing is human imagination. We think if we conceive of it then it must exist.

The same logic, or imaginary power, applies to the existence of any mythological creature. Actually, every single mythological creature—even space aliens—is just a combination of multiple animals people have first hand experience with; that is, we "copy" and then "paste" the parts of one animal on to another animal to create a new concept, e.g. in Thale's time harpy's and the Chimera were believed real. The concept of a harpy most certainly can exist but short of some significant cosmetic surgery a "hawk lady" is just not possible.

harpy and chimera

In the case of aliens, just think of the way they are represented in film, i.e. they're essentially "little green men" who just look like a weird version of us. Logically speaking, why would aliens necessarily look like us? The funny thing is on the planet earth there are stranger looking things than aliens when compared to humans. For example, consider how different snakes and viper fish are compared to humans; it stands to reason then that an alien, a being from another planet millions of light years distant, is going to be at least as different as a viper fish is compared to a human being (if not more so); moreover, it is an assumption that alien life even exists.

In conclusion, producers of alien movies suffer from the same problem as scholastics like Anselm: scholastics deal from concepts but not from reality. The philosophy of humanism, which William of Ockham most certainly influenced, gave people better logical tools and methods of testing reality.

Secondly, William argued that the creative capabilities of the human mind made people capable of imagining things in to existence that did not exist; that being, opinions might exist on the ability of a witch to cast a spell but that didn't mean witches could actually cast spells. Therefore, a concept or idea was only evidence of a particular way a person thought about the world, e.g. I am a person who believes witches can cast spells.

Again, a concept is not a "thing" in and of itself; and things do not need us to believe in them to exist. In the end, a concept is just an opinion; and the existence of opinions—or ideas—are not evidence of anything other than the existence of a thought bumping around in a person's mind; and opinions can exist on things that do not.

William of Ockham was not an atheist. He was a Catholic monk. He also wasn't even a Renaissance writer and thinker; however, his work anticipated both the Renaissance and the Scientific Revolution. This is because he was one of the first scholars (in a long line including Galileo Galilei, Isaac Newton, and eventually, Albert Einstein) who wanted to see reality as it actually was as opposed to how it was believed to be.