— HermanS

Hello! Apologies for the late reply. Here are the places where I think you went wrong. All of these suffer from the same problem it seems:

In the 2 examples for 6.6.2, you sent both negative conditionals from world 0 to world 1. But this is incorrect: each negative conditional should bring a new world into the tableau. So, for instance, you can send (P -> Q), -0 to world 1, but now you have to send (Q -> S) -0 to a new world, say world 2). Give that a shot and see how it goes :smile:

Now for exercise 6.10.4c there's a similar problem, when you negate a negative proposition, you need to bring a new world to the tableau. But you sent both of them to the same world. Therefore, after sending -P, -0 to world 1, you need to send -Q, -0 to world 2.

Same thing with 6.10.4e. You sent both negative conditionals to world 1, but you need to send the first to world 1 and the second to world 2.

I hope this was helpful. Please let me know if you have more questions.