For this achievement you have to clear the thoughts you've collected several times. You collect thoughts by pressing X when you are near an item with a thought bubble. You can view your collected thoughts by pressing M. While in this view press and hold C until all the thoughts clear. Since tapping C will close this view it's a good idea to hit M again and verify the thoughts cleared.
I actually got this achievement later in the game by returning to the Golden Gate - since everything in there is constantly producing thoughts it was easy to collect several and then clear them. If you've already been to the gate (it's the last stage of the tutorial) you can transform an object to the Golden Gate and enter it that way. Transform an object by hitting E and going to Other > Golden Gate.