Ad ignorantum
The argument from ignorance basically states that a specific belief is true because we don't know that it isn't true. Defenders of extrasensory perception, for example, will often overemphasize how much we do not know about the human brain. It is therefore possible, they argue, that the brain may be capable of transmitting signals at a distance.
UFO proponents are probably the most frequent violators of this fallacy. Almost all UFO eyewitness evidence is ultimately an argument from ignorance – lights or objects sighted in the sky are unknown, and therefore they are alien spacecraft.
Intelligent design is almost entirely based upon this fallacy. The core argument for intelligent design is that there are biological structures that have not been fully explained by evolutionary theory, therefore a powerful intelligent designer must have created them.
Confusing absence of evidence with evidence of absence
This fallacy cuts both ways. In other words, one might assume that absence of evidence is a compelling argument against the reality of a claim or phenomenon. On the other hand, however, one might dismiss the absence of evidence as having no significance – as not being evidence of absence.
In reality, the absence of evidence can only be properly considered in the context of how likely it is that evidence should exist. You must therefore consider how thoroughly evidence has been looked for, and if the tools and techniques employed are capable of finding evidence. For example, if you dredge the bottom of a sea with a net that has one inch holes in order to survey the life living in that sea, you will likely not come up with any creatures smaller than one inch in size. It would not be logical to then conclude that there are no fish smaller than one inch living in that sea.
This fallacy is central to the Bigfoot debate. Believers in Bigfoot claim that the absence of hard evidence of Bigfoot is not evidence that Bigfoot does not exist. Skeptics, however, argue that a population of large mammals living in North America should produce some evidence, such as the occasional corpse.
