Dissoi Logoi (Greek δισσοὶ λόγοι "contrasting arguments") is a rhetorical exercise of unknown authorship. Based on comments in the text it appears to have been written not long after the Peloponnesian War. It is intended to help an individual gain a deeper understanding of an issue by forcing them to consider it from the angle of their opponent, which may serve either to strengthen their argument or to help the debaters reach compromise.
In ancient Greece, students of rhetoric would be asked to speak and write for both sides of a controversy. The Dissoi Logoi was found appended to a manuscript of the works of the Pyrrhonist philosopher Sextus Empiricus. It was first published by Stephanus in 1570, as an appendix to his edition of Diogenes Laertius, and it is found here divided into five chapters. Thomas Gale first published a version of it with a commentary of its own, in 1671. The first edition with an apparatus criticus was published by Ernst Weber in 1897.
The composition date of the work is unknown. Scholars look to the text to piece out clues as to its origin, but find that even this is ambiguous at best. One possible way to date the work is its mention of the offspring of Polyclitus, a well known Greek sculptor. In the Dissoi Logoi (section 6.8) it states that Polyclitus taught his son (singular) the virtue of ἀρετή. Yet in Protagoras 328c, the usually attentive Plato claims Polyclitus to have in fact two sons, not just one. The Protagoras's dramatic events are conventionally dated to between 429 and 422 BC, so either one of the authors made a mistake in listing the genealogy of Polyclitus, or the Dissoi Logoi was written before Polyclitus had another son, thus dating it to before the 420s BC.
Another interesting reference possibly dating the text is its mention of a victory of Sparta over Athens and her allies at section 1.8. At face value, most tend to accept this as a reference to the Peloponnesian War, and thus claim that the Dissoi Logoi must have been written after this war's terminal date, 404 BC. While this is most probably true, it is by no means sure, because there are other instances of Spartan victory over Athens which add uncertainty to this dating, such as the Battle of Tanagra in 457 BC. Thus the Dissoi Logoi are generally dated to between the 5th and early 4th centuries BC.
Dissoi Logoi, also called dialexeis, is a two-fold argument, which considers each side of an argument in hopes of coming to a deeper truth. The dissoi logoi doctrine provides historical insight into early sophistic rhetoric. Silvermintz notes that while dissoi logoi purports to offer a consideration of both the absolutist and relativist positions, the latter chapters defending the sophists demonstrate its allegiance to the relativist position. It is similar to a form of debate with oneself and holds that contradiction is an inevitable consequence of discourse. Rhetorician John Poulakos sees dissoi logoi as the ability or practice of providing a contrary argument at any point on any issue. He says that people must be persuaded to one side or the other in order to act, and this is accomplished through dissoi logoi.
It considers demonstrating contrasting arguments in a single oration a method of demonstrating skill. Protagoras stated that every argument had two contradicting sides, both of which could be argued. This idea emphasizes the power and versatility of language. A definite parallel can be drawn between the thoughts of Protagoras as recounted by Plato, and the rhetorical methods used in the Dissoi Logoi. Aristotle supported this idea by stating that it is necessary to think in opposites to anticipate counterarguments and to arrive at the true state of an idea, object, etc. He also states that what is beneficial for one group might not necessarily be advantageous for another. Dissoi Logoi considers that rhetoric can be situational. In regards to Gorgias, the persuasive argument is dependent upon what seems logical according to the situation at any given time. Good and bad are relative to context, point of view, time, place, etc. Edward Schiappa further simplified the concept of dissoi logoi by putting the concept into the following form: "X can be Y and not-Y." 
What cannot be denied is the confounding nature with which the Dissoi Logoi conveys its message. For instance, in the very first chapter, the author states "some say that what is good and what is bad are two different things, others that they are the same thing..I myself side with the latter group", yet by the end of this chapter, it has changed "I am trying rather to point out that it is not the same thing which is bad and good, but that each is different from the other Rosamond Sprague argues that good and bad cannot be the same and are, in fact, different from each other. He exemplifies this by looking at the concept of war. If good and bad were the same, then by doing a great deal of harm, you would also be doing them the greatest goods. On the true purpose of the Dissoi Logoi one scholar writes "it could be a serious, and hence disappointingly bad treatise; a heavy-handed spoof of such (Sophist) works; a workbook for dialecticians...It is almost impossible to say anything about the Dissoi Logoi that goes beyond mere conjecture." 
There are many 5th and 4th century BC works that touch upon similar concepts mentioned in the Dissoi Logoi. As previously mentioned, the Dissoi Logoi's attempt to argue and issue from both sides is reminiscent of Plato's Protagoras, which was presumably written after the Dissoi Logoi. It could be that the Dissoi Logoi could have been derived from Protagoras himself, and may have even been an influence on Plato while he was writing his Protagoras.
The Dissoi Logoi speak in detail about the acquisition of language in humans, which is ultimately determined to be learned, not inherent (6.12). He comes to this conclusion through a question, "What if a Greek child is born in Greece and immediately sent to live in Persia? The answer of course is that the child would speak Persian, not Greek, and therefore language must be learned. A similar debate is waged in Herodotus's Histories 2.2, where an Egyptian king, Psammetichus, attempts to determine the world's first language by raising two newborns completely in lack of language. The children independently begin to speak Phrygian, which is then determined to be the first language of man. Both Herodotus, and the author of the Dissoi Logoi seem to have invested thought into the developments of language 
- "An Introduction to the Dissoi Logoi". Northern Illinois University. Retrieved 2008-11-26.
- Robinson, T.M. "Contrasting Arguments" pp.1-15. Arno Press. 1979
- Robinson, T.M. "Contrasting Arguments" pp.34-35. Arno Press. 1979
- Robinson, T.M. "Contrasting Arguments" pp.35-37. Arno Press. 1979
- Bailey, D.T.J. 2008. "Excavating the Dissoi Logoi 4". Oxfort Studies in Ancient Philosophy: 249-64
- Sprague, Rosamond (April 1968). "Dissoi Logoi or Dialexeis" (PDF). 77 (306): 6. Retrieved 7 October 2013.
- Daniel Silvermintz (2008). "The Double Arguments". In Patricia O'Grady. The Sophists: An Introduction. London: Duckworth Academic. pp. 147–153.
- Hawk, Byron (2007). A Counter-History of Composition: Toward Methodologies of Complexity. Pittsburgh, PA: University of Pittsburgh.
- Gera, D.L. Two Thought Experiments in the Dissoi Logoi.The American Journal of Philology121(1): 21-45
- Burton, Gideon O. "Figures of Opposition". Brigham Young University. Retrieved 7 October 2013.
- Higgins, Francis. "Gorgias". Internet Encyclopedia of Philosophy. Retrieved 22 October 2013.
- Schiappa, Edward (2005). Classical Rhetorics and Rhetoricians: Critical Studies and Sources. Westport, CT: Greenwood Publishing Group, Inc. pp. 146–148.
- Robinson, T.M. "Contrasting Arguments" pp.99-105. Arno Press. 1979
- Sprague, Rosamond Kent. "Dissoi Logoi or Dialexeis-Two-Fold Arguments" (PDF). Oxford University Press. Retrieved 16 Feb 2014.
- Bailey, D.T.J. 2008. "Excavating the Dissoi Logoi 4". Oxford Studies in Ancient Philosophy: 250'
- Gera, D.L. Two Thought Experiments in the Dissoi Logoi.The American Journal of Philology121(1): 24
- Gera, D.L. Two Thought Experiments in the Dissoi Logoi.The American Journal of Philology121(1): 25
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