The Roman Emperor and Stoic philosopher Marcus Aurelius coined many a memorable phrase, including: "Be not careless in deeds, nor confused in words, nor rambling in thought."
Thankfully, the Members of Dáil Éireann can avoid being confused about what they can and can't say by reading the Standing Orders.
The document we're interested in is entitled: Salient Rulings of the Chair, Fourth Edition, 2011. It runs to 78 pages, as it contains rulings dating back decades.
My interest in this was piqued by Taoiseach Micheál Martin who used the phrase "ag insint bréaga" when sparring on housing policy with the Sinn Féin leader Mary Lou McDonald yesterday.
According to my sources, fluent as Gaeilge, the phrase translates as "telling lies".
The truth is, however, it's unclear whether or not this crosses a line in parliamentary terms.
Ceann Comhairle Verona Murphy, who isn't a fluent Irish speaker, now has the responsibility of making a ruling, on foot of a complaint by the Sinn Féin leader.
But let's have a look at what she will base any ruling on (or, if we're being pedantic, on what she will base any ruling on.)
Things get interesting about page 40 of the Standing Orders, when we reach Chapter 12: "Personalities, personal charges and threats."
It starts off with the obvious: "Section 411. Members must not threaten another Member."
Then there is: "413. If an allegation impugns the character, integrity and good name of a Member it must be withdrawn."
Now it gets into the weeds where some of the text sounds like it's lifted from a novel about Irish highwayman 'Captain' Gallagher in the late 1700s.
Under the heading "Allegations or reflections of this nature which have been ruled as disorderly", we learn that it's out of order to claim another Member is guilty of: "murder or condoning or conniving at murder; blackmail; corruption, corrupt practices; and perjury."
Also ruled out are: "accusations of physical or moral cowardice; graft, embezzlement or malversation [corrupt behaviour in a position of trust] and defalcation [misappropriating by a person in charge] of public funds; robbery; seditious libel; and claims of interfering in the distribution of land while a Minister of State at the Department of Agriculture" (No other department is mentioned here).
It goes on to list some choice phrases that Members can't use against their parliamentary colleagues.
A Member can't say of another Member that they're: "dishonest; a black-marketeer; or a smuggler."
Who knew that a Member can't accuse another of being: "a rogue; scoundrel; protector of thieves and rogues; or trying to put his hand in the public purse."
Here's where it might get difficult for Micheál Martin under Section 422, which states: "Every Member states what is known to him* as the truth and it is on that basis that debates are conducted." (* language needs updating on gender grounds!)
Accordingly, the Standing Orders definitively rule out a Member accusing another of being: "a liar, lying or telling a lie; telling untruths or not telling the truth; deliberately misstating what was said."
It seems, therefore, highly likely that the Taoiseach will have to retract his accusation of "ag insint bréaga".
But as one Fianna Fáil TD suggested to me, while brandishing his foclóir Gaeilge-Béarla, the problem is that the Irish language doesn't have a word for "untruth".
I don't think that's going to hold much uisce with the Ceann Comhairle.
There's more. Section 428 of the Standing Orders protects Members from being subject to "... insulting and abusive expressions".
These include: "brat, acting the brat; buffoon, buffoonery; chancer; communist; corner boy (or girl?), corner boy tactics; coward; fascist, fascist minister; gurrier; guttersnipe; hypocrite, bloody hypocrites, hypocrisy."
Other banned insults include: "Rat; scumbag; scurrilous, scurrilous speaker; and yahoo."
Maybe, just maybe, the Ceann Comhairle might let the phrase "yahoo" through without objecting.