I have Brain in a Jar with 0 counters on it. Opponent has Teferi, Time Raveler on his side of the field.
I put a counter on Brain in a Jar and attempt to play Unsummon.
Opponent says I can't because that I'm not casting it as a sorcery.
I am confused, does Brain in a Jar only cast at instant speed even during my turn?
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-MadMage
There are two different things at play here and it is important to understand both to understand how they interact with one another.
First, Brain in a Jar. Brain in a Jar gives you permission to cast a spell during the resolution of the ability. You can tell this because there is no duration given (such as "this turn" or similar). So, you cast the spell as the ability is resolving or you don't cast it at all. This is important because this a time when *no* spells can be cast. Not even instants. Spells cannot be cast in the middle of another spell or ability is resolving unless that spell or ability explicitly gives you permission to do so.
Also, and this is important for Teferi's ability, there is nothing in the rules that says you *can't* cast a Sorcery on an opponent's turn. The rules are not written this way. Instead, the rules are written to say when things *can* happen. Thus, the rules for casting a Sorcery are "when it is your main phase, the stack is empty, and you have priority". Brain in a Jar adds to this permission to make the permission for casting a Sorcery (for example): "[when it is your main phase, the stack is empty, and you have priority] or [when Brain in a Jar's ability is resolving]". This does not contradict or supersede the rules because it is simply adding to what the rules already allow to give you an additional moment you can do something.
Now, on to Teferi. Teferi is different. Teferi explicitly says you *can't* do something ("can only" is the same thing as "can't"). This means he does introduce an actual restriction. This restriction is enforced on top of Brain in the Jar's ability because of rule 101.2:
101.2. When a rule or effect allows or directs something to happen, and another effect states that it can't happen, the "can't" effect takes precedence.
Because Teferi is a "can't" effect, he takes precedence and will prevent the spell from being cast via Brain in a Jar because Brain in a Jar is trying to make you cast the spell at a time when you can't normally cast a Sorcery. That is, when an ability is resolving.
This also brings us into other cards, such as Cauldron Dance. This card has a built in restriction for when it can be cast so that restriction will also supersede the permission being granted by Brain in a Jar for the same reason Teferi does. It is telling you that you can't do something so you can't.
Without Teferi, or an "on-card" restriction like Cauldron Dance, Brain does indeed let you cast something like Time Wipe on your opponent's turn because Brain in a Jar is giving you permission to do so and nothing else is taking that permission away.
I put a counter on Brain in a Jar and attempt to play Unsummon.
Opponent says I can't because that I'm not casting it as a sorcery.
I am confused, does Brain in a Jar only cast at instant speed even during my turn?
Using card tags is mandatory in the Magic Rulings forum. Please check your private messages to learn how to use them. I've added them for this post.
-MadMage
Brain in a Jar
Teferi, Time Raveler
There are two different things at play here and it is important to understand both to understand how they interact with one another.
First, Brain in a Jar. Brain in a Jar gives you permission to cast a spell during the resolution of the ability. You can tell this because there is no duration given (such as "this turn" or similar). So, you cast the spell as the ability is resolving or you don't cast it at all. This is important because this a time when *no* spells can be cast. Not even instants. Spells cannot be cast in the middle of another spell or ability is resolving unless that spell or ability explicitly gives you permission to do so.
Also, and this is important for Teferi's ability, there is nothing in the rules that says you *can't* cast a Sorcery on an opponent's turn. The rules are not written this way. Instead, the rules are written to say when things *can* happen. Thus, the rules for casting a Sorcery are "when it is your main phase, the stack is empty, and you have priority". Brain in a Jar adds to this permission to make the permission for casting a Sorcery (for example): "[when it is your main phase, the stack is empty, and you have priority] or [when Brain in a Jar's ability is resolving]". This does not contradict or supersede the rules because it is simply adding to what the rules already allow to give you an additional moment you can do something.
Now, on to Teferi. Teferi is different. Teferi explicitly says you *can't* do something ("can only" is the same thing as "can't"). This means he does introduce an actual restriction. This restriction is enforced on top of Brain in the Jar's ability because of rule 101.2:
Because Teferi is a "can't" effect, he takes precedence and will prevent the spell from being cast via Brain in a Jar because Brain in a Jar is trying to make you cast the spell at a time when you can't normally cast a Sorcery. That is, when an ability is resolving.
This also brings us into other cards, such as Cauldron Dance. This card has a built in restriction for when it can be cast so that restriction will also supersede the permission being granted by Brain in a Jar for the same reason Teferi does. It is telling you that you can't do something so you can't.
Without Teferi, or an "on-card" restriction like Cauldron Dance, Brain does indeed let you cast something like Time Wipe on your opponent's turn because Brain in a Jar is giving you permission to do so and nothing else is taking that permission away.