Question: **Agrippa's 5 tropes**

1. *Dissent*: For every thesis an anithesis
2. *Relation*: Point of view matters.
3. *Infinite Regress*: Justifications need justifications need justifications ... *ad infinitum*
4. *Axioms*: Assume something to be true *sans* justification.
5. *Circularity*: The conclusion appears in the premises.

3, 4, 5 (above), together, constitute the well-known *Agrippa's trilemma*.

One *seemingly* strong argument against skepticism has been to point out that it's a *peritrope* (self-refuting). However, for it to be so, it *must be a sound argument* and if that's the case the argument that skepticism is a peritrope is itself a peritrope and this further argument I just made is also one ... *ad infinitum*. Skepticism is unsound only if skepticism is sound.

We end in a rather mind-boggling contradiction

1. For skeptics, skepticism is a good argument implies it's a *peritrope*.
2. For dogmatists, skepticism is a *peritrope* implies it's a good argument.

Skepticism → Peritrope → Skepticism → ... ad infinitum/ad nauseum

How do we extricate ourselves from this quagmire?

Answer: 
As per [my other answer](https://philosophy.stackexchange.com/a/92305/14508), **a skeptical argument is not different than an assertion of the presence or lack of presence of something**.

For example:

> 
> A: I see a boat out there.
> 
> 
> B: How do you know you see a boat out there?
> 
> 
> 

What B is doing is asserting something, even if only indirectly. B asserts that A's sense (at that point) is incorrect and/or doubtful. But **this is an assertion**, and **an assertion needs to be based on something**, even if it is on a self-evident something.

So extracting the assertion out of a skeptical argument, makes it like any other argument and can be attacked or refuted all the same. One can simply ask the other to ground the implicit assertion the person makes on something, even on a self-evident something.

If one denies that an assertion needs to be based on something, then the skeptic *loses the teeth to bite*, since now the person cannot attack any statement made.

PS: That an assertion needs to be based on something holds for person A too. Possibly A can appeal to the self-evident fact of being a boat there, at least as a starting point.

**Is skepticism invincible?**

1. A skeptical argument or challenge, as argued above, is like any other argument or statement and can be attacked or refuted all the same. Nothing invincible in that.
2. Skepticism as a whole is nothing but the set of potential skeptical arguments or challenges. Nothing invincible in that either.

Note: you may be interested in [this post](https://philosophy.stackexchange.com/questions/81795/is-there-any-counterexample-given-against-radical-skepticism/92321#92321) about *radical* skepticism.
Also you may be interested in the [solution(s) to the problem of infinite regress in epistemology](https://philosophy.stackexchange.com/a/93954/14508)
**Agrippa's 5 tropes resoleved**

1. *Dissent*: Trust an other as thyself and you won't need the theses. An other is not an alien or evil, don't be mad in faith.
2. *Relation*: Point of view doesn't matters for alitheya, be at skeptic's point.
3. *Infinite Regress*: Not needed an justification if you are not an other-I.
4. *Axioms*: Art the axioms needed by an action in one place and one time. Axioms are something here and now only, not there and when.
5. *Circularity*: Don't use refers, break the vicious circle.