meaning of "Error by his own arms is best evinced" ?

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Did you try first locating the text and reading [url=http://www.archive.org/stream/miltonworks02miltuoft/miltonworks02miltuoft_djvu.txt]A BIT OF IT[/url]?

And with the Gentiles much thou must converse,
Ruling them by persuasion, as thou mean'st.

haha, no i thought it was a quote on its own..anyway i still do not get what it means..

what i make out is "***'s should express his mistakes?" , or is it more of "satan should accept his mistakes?"

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More like 'Mistakes are best revealed by considering the mistakes themselves'.

eirelavError by his own arms is best evinced.

The best way to evince (reveal) errors is with the arms (weapons) of the errors themselves.
It's a metaphoric way to say
The best way to show someone that he's in error is to use his own way of seeing the world as a weapon against him. Take his argument and elaborate it, showing what it implies, sh

CalifJimTake his argument and elaborate it, showing what it implies, showing what absurd conclusions would result. He will then realize his error.

Sadly, as you no doubt know, this works far less often than one might hope. Far more often than not one is met by obstinate rejection of the argument and a change to an entirely different line of attack.

RayHSadly, as you no doubt know, this works far less often than one might hope. Far more often than not one is met by obstinate rejection of the argument and a change to an entirely different line of attack.

You'll have to take that up with Milton!

You're all not quite right. Satan is saying that because the Greek philosophers have arrived at false conclusions using reason, the best way to correct them is to use reason against them, their own tool.