Does Rule of Law prevent you from casting spells with flash during an opponent's turn?

Rule of Law (and the functionally identical Arcane Laboratory) states that each player can't cast more than one spell each turn. This is a hard restriction on the number of spells cast in a turn, not on the timing of when they are cast. Flash (CR 702.8) only modifies when you may cast a spell — it lets you cast at instant speed — but it does not override other restrictions on casting spells (CR 601.3).

CR 601.3 specifies that a player must follow all applicable rules and effects when casting a spell. Rule of Law's restriction applies to the casting process itself, so even if you have already cast a spell during your opponent's turn using flash, you cannot cast a second spell that same turn under Rule of Law's constraint.

It is important to note that 'each turn' in Rule of Law refers to each player's individual turn. So if your opponent casts a spell on their turn and you respond with a flash spell, you have used your one spell for their turn. You cannot then cast another spell (even with flash) during that same opponent's turn. However, on your turn, your one-spell allowance resets independently.

Example: Rule of Law is on the battlefield. During your opponent's turn, you cast Snapcaster Mage using its flash ability. Later that same turn, your opponent plays another spell and you want to respond with Counterspell. You cannot — you already cast one spell this turn, and Rule of Law prevents a second, regardless of flash.

CR references: Flash is defined in CR 702.8a; the general casting restriction interaction is governed by CR 601.3, which requires all applicable restrictions be met; Rule of Law's effect functions as a continuous effect imposing a rule under CR 613.