Agrippa was a Skeptic philosopher who probably lived towards the end of the 1st century AD. He is regarded as the author of "five grounds of doubt" or tropes , which are purported to establish the impossibility of certain knowledge.

The Five Tropes

These tropes are given by Sextus Empiricus

Sextus Empiricus

Sextus Empiricus , was a physician and philosopher, and has been variously reported to have lived in Alexandria, Rome, or Athens. His philosophical work is the most complete surviving account of ancient Greek and Roman skepticism....

, in his Outlines of Pyrrhonism. According to Sextus, they are attributed only "to the more recent skeptics" and it is by Diogenes Laertius

Diogenes Laertius

Diogenes Laertius was a biographer of the Greek philosophers. Nothing is known about his life, but his surviving Lives and Opinions of Eminent Philosophers is one of the principal surviving sources for the history of Greek philosophy.-Life:Nothing is definitively known about his life...

 that we attribute them to Agrippa. The tropes are:

1. Dissent
   Dissent
   Dissent is a sentiment or philosophy of non-agreement or opposition to a prevailing idea or an entity...
   
   – The uncertainty of the rules of common life, and of the opinions of philosophers.
2. Progress ad infinitum
   Ad infinitum
   Ad infinitum is a Latin phrase meaning "to infinity."In context, it usually means "continue forever, without limit" and thus can be used to describe a non-terminating process, a non-terminating repeating process, or a set of instructions to be repeated "forever," among other uses...
   
   – All proof requires some further proof, and so on to infinity.
3. Relation – All things are changed as their relations become changed, or, as we look upon them from different points of view.
4. Assumption
   Assumption
   In logic an assumption is a proposition that is taken for granted, as if it were true based upon presupposition without preponderance of the facts...
   
   – The truth asserted is merely a hypothesis.
5. Circularity
   Begging the question
   Begging the question is a type of logical fallacy in which the proposition to be proven is assumed implicitly or explicitly in the premise....
   
   – The truth asserted involves a vicious circle.

[165] According to the mode deriving from dispute, we find that undecidable dissension about the matter proposed has come about both in ordinary life and among philosophers. Because of this we are not able to choose or to rule out anything, and we end up with suspension of judgement

Epoché

Epoché is an ancient Greek term which, in its philosophical usage, describes the theoretical moment where all judgments about the existence of the external world, and consequently all action in the world, is suspended...

. [166] In the mode deriving from infinite regress, we say that what is brought forward as a source of conviction for the matter proposed itself needs another such source, which itself needs another, and so ad infinitum, so that we have no point from which to begin to establish anything, and suspension of judgement follows. [167] In the mode deriving from relativity, as we said above, the existing object appears to be such-and-such relative to the subject judging and to the things observed together with it, but we suspend judgement on what it is like in its nature. [168] We have the mode from hypothesis when the Dogmatists, being thrown back ad infinitum, begin from something which they do not establish but claim to assume simply and without proof in virtue of a concession. [169] The reciprocal mode occurs when what ought to be confirmatory of the object under investigation needs to be made convincing by the object under investigation; then, being unable to take either in order to establish the other, we suspend judgement about both.

With reference to these five tropes, that the first and third are a short summary of the ten original grounds of doubt which were the basis of the earlier skepticism. The three additional ones show a progress in the skeptical system, and a transition from the common objections derived from the fallibility of sense and opinion, to more abstract and metaphysical grounds of doubt.

According to Victor Brochard “the five tropes can be regarded as the most radical and most precise formulation of skepticism that has ever been given. In a sense, they are still irresistible today.”

See also

• Aenesidemus
  Aenesidemus
  Aenesidemus was a Greek sceptical philosopher, born in Knossos on the island of Crete. He lived in the 1st century BC, taught in Alexandria and flourished shortly after the life of Cicero...
  
  
• Münchhausen Trilemma
• Philosophical skepticism
  Philosophical skepticism
  Philosophical skepticism is both a philosophical school of thought and a method that crosses disciplines and cultures. Many skeptics critically examine the meaning systems of their times, and this examination often results in a position of ambiguity or doubt...
  
  
• Pyrrhonism
  Pyrrhonism
  Pyrrhonism, or Pyrrhonian skepticism, was a school of skepticism founded by Aenesidemus in the 1st century BCE and recorded by Sextus Empiricus in the late 2nd century or early 3rd century CE. It was named after Pyrrho, a philosopher who lived from c. 360 to c. 270 BCE, although the relationship...
  
  
• Sextus Empiricus
  Sextus Empiricus
  Sextus Empiricus , was a physician and philosopher, and has been variously reported to have lived in Alexandria, Rome, or Athens. His philosophical work is the most complete surviving account of ancient Greek and Roman skepticism....
  
  
• Theory of justification
  Theory of justification
  Theory of justification is a part of epistemology that attempts to understand the justification of propositions and beliefs. Epistemologists are concerned with various epistemic features of belief, which include the ideas of justification, warrant, rationality, and probability...