Greek Logic is a puzzle where each symbol must appear exactly once in each row, column, and main diagonal. In the picture below, the symbols on the top row, are from left to right: Phi, Delta, Theta, Pi.
First, notice that in the 3rd column, Pi can only go in the 4th row, as all other placements would cause conflicts.
After placing this Pi, there is only one row and one column lacking Pi, the 3rd row 1st column.
Next, notice that in the 3rd column, Theta can only go in the 2nd row, as placing it in the 3rd row would cause a conflict.
Now the situation with Theta looks exactly the same as our initial situation with Phi, but mirrored, and so all placements of Theta have been found.
Next, the top left to bottom right diagonal, and the 3rd column, each are lacking exactly one symbol, so fill those in.
Then the rest of the puzzle can be completed, using the strategies already identified.