Skepticism has a long history dating back to the 5th century B.C. with the sophists and to the 4 century B.C. with the Pyrrhonians.^[1] This article will look at these two ancient forms of skepticism for historical context and then look at four different versions of modern skepticism.
 

Ancient Schools of Skepticism
Two main schools of skepticism thrived in the 5th and 4th centuries BC:

1. Academic skepticism--two main assertions:^[2]
   1. The skeptical thesis: all things are inapprehensible, therefore, no one has any knowledge.
   2. Pertaining to the skeptical thesis: the skeptic dogmatically affirms that we know that no one has any knowledge. (italics mine)
2. Pyrrhonian skepticism--two main assertions:^[3]
   1. Reject dogmatism.
   2. Suspend judgment about all things including the skeptical thesis (Academic skepticism's first assertion).

    

 Academic Skepticism (AS)
Assertion a: all things are inapprehensible, therefore, no one has any knowledge.
Academic skepticism (AS) has some difficult obstacles to overcome. Notice that the skeptical thesis is something that can, supposedly, be know (even though it claims no one has any knowledge, emphasis mine). If this is correct, it is hard to see how the skeptical thesis couldbe known by the skeptic since this is a claim to knowledge while, at the same time, we are not suppose to have any knowledge! As such, the AS would have a hard time even getting off the ground.

However, it could be that the skeptical thesis (ST) actually claims to have one piece of knowledge, namely, that all things are inapprehensible, therefore, no one has any knowledge.^[4] In other words, the ST is the single piece of knowledge a person has. But, if this is the single piece of knowledge one has, then it becomes problematic since claiming “all things are inapprehensible” other than the ST implies that the skeptic knows that there is more than one thing to know (more than simply the ST). Yet according to the ST the only thing known is the ST. Thus, the STappears contradictory.

It seems that no matter which interpretation embraced regarding assertion "a" it remains untenable.

Assertion b: the skeptic dogmatically affirms that we know that no one has any knowledge. (italics mine)
As one considers the second assertion of the AS it becomes apparent that this is simply self-refuting. To dogmatically affirm that one knows that no one can know anything is contradictory. One simply has to ask, “How can one know ‘no one has any knowledge?’” Since, according to the AS, this is unknown no one can know this. This second assertion is a claim to knowledge while at the same time claiming that no one has any knowledge. This seems more like a bullheaded belief than a sound one. Due to its self-refuting nature, AS should be abandoned.

Pyrrhonian Skepticism (PS)
Assertion a: reject dogmatism
Pyrrhonian skepticism’s (PS) first assertion is to reject dogmatism. If the PS assertion (reject dogmatism) is to be taken seriously, then it too seems dogmatic. If so, it is hard to see how dogmatism has been rejected. Dogmatism has simply been refocused and still accepted rather than actually rejected. Instead of being dogmatic that we can know our own thoughts, the PS is dogmatic that we should not be dogmatic. This glaring problem continues to plague many skeptics today.

Assertion b: Suspend judgment about all things including the skeptical thesis (Academic skepticism's first assertion).
PS’s second assertion is to suspend judgments about all things including the skeptical thesis.^[5] This results in an agonistic view (basically, the belief that a view can be neither proved nor disproved) about everything. It seems difficult to see how this can actually be lived out. For instance, how is a mother supposed to suspend all judgment when she sees her young son drowning? Or, how is a jury supposed to be agnostic about the guilt or innocence of a murderer especially with overwhelming evidence?

While suspending judgment can be noble at times (the jury should suspend judgment prior to hearing all evidence from both sides), suspending judgment is not a doctrine that can be easily or effectively lived out at all times. However, a more subtle problem arises with the PS assertion that all judgment should be suspended about all things including the skeptical thesis. This view is itself a judgment that is not suspended. Therefore, one is left doing the very thing that one is not suppose to do, namely, making a judgment.