Suppose a lawyer has made an argument in one case based on one legal principle or assertion of facts. Can he assert the exact opposite in a different case involving a different client?
Yes. Otherwise, wouldn’t you eventually run out of arguments to use?
If my client is a landlord who has a tenant that moved out before the end of the lease, I’m going to argue that the tenant still owes rent because he signed a lease for the full term, and that the contract empowers the landlord to enforce this obligation. The tenant has no argument to make about the condition of the apartment because it was like that when he took possession.
But if I then represent a tenant who moved out before the end of the lease, I’m going to argue that the landlord’s behavior forced the tenant to move (“constructive eviction”) and that the landlord is obligated to re-rent the apartment without waiting for the tenant to pay (“self help”)
Lawyers advocate for their client’s best position; that’s what they do.
ETA: estoppel applies to litigants.
I had actually assumed that estoppel doesn’t apply to lawyers, and was just double-checking.
But to be clear, the circumstances I’m thinking of are not the situation you describe. In your case, you seem to be discussing a completely separate set of facts in two different incidents. The situation I’m thinking of is where you allege two different versions of the same exact facts, in two different cases.
For example, suppose you represent a guy accused of causing an accident on such-and-time in such-and-such place, and your argument is that visibility was very poor which lessened his fault (let’s assume that this is legally correct, at least in certain circumstances). Then you represent - in a completely separate case - a guy who got a traffic ticket for driving too fast for the poor visibility in that exact time and place, and in that case you argue that visibility was perfectly fine and the guy was entitled to drive normally.
Or similarly if there’s some legal position which has some ambiguity to it, and you argue one side of the legal issue in one case and then take the exact opposite position that same legal issue in another case.
Now that I think about it, I once read about a case where prosecutors put one guy on trial for committing some crime and then later put a different guy on trial based on an alternative version of events in which the second guy was the (primary?) criminal. I imagine one of the cases discussed here. I imagine that itself may settle the issue.
No two cases ever are exactly equivalent. There will always be different facts and circumstances.
Absolutely no problem with the two situations you discuss. Remember, the lawyer isn’t arguing what he thinks, he’s arguing what the evidence might support. There is (almost) always different ways to argue the evidence. Using your visibility example, there are facts present where an argument could be made that the visibility was a problem. And, for the next case, facts from which someone could argue that visibility was fine. It doesn’t matter what the lawyer’s opinion on the visibility is. The lawyer isn’t an expert, or even a witness. He or she is just arguing from the evidence.
What Exapno Mapcase and Procrustus said. At the granular level, there are no cases that are the same, and it is at the granular level that cases are conducted. You can’t idealise similar cases together because matters of detail will make a difference. The credibility of the witnesses will always differ, for example, as will their opportunities to see things, etc. Similarity (at some level of abstraction from the detail) is not identity.
A lawyer’s arguments are of several different types (this is not a closed list).
First, there is the base level argument about whom to believe in cases of testimonial conflict. That obviously will be highly specific to the particular case at hand.
Second, there are arguments of law, such as “Does the postal rule apply here?” This too is dependent on the particular facts of any given case.
Thirdly, there are arguments of law of the sort that get a run in ultimate courts of appeal such as SCOTUS, etc. “The postal rule is no longer good law.” Such an argument is at a level of abstraction a long way from the particular facts, but the court will always want to be persuaded that the case at hand is a good vehicle factually for reconsidering the postal rule. Even at this level, however, a lawyer can argue one way one day in one court, and another way another day in another court in a different case. Note, however, that their argument will change between cases depending on the outcome of the first case, which by the time of the second case will now be at least persuasive authority one way or another.
And lastly, lawyers are not prohibited from arguing contradictory propositions in the alternative. “The postal rule is no longer the law, but if I am wrong about that, applying its principles means that it has no application on the facts of the case here.” The first argument proceeds on the premise that the postal rule is bad law, the second assumes it is good law.
Counsel may not, however, represent two different sides of the same case, as you recognise. I had a case in the High Court of Australia where on the morning of the argument, an unexpected barrister turned up to argue it. It was a case with a long history, and the unexpected barrister had been the one who had appeared in court to argue it for my side some years previously. The court suggested there was substance in my objection to his appearance and invited the other side to get another barrister who was not compromised.