Is it possible to have an original thought?

Many of our thoughts are reactions to a singular external stimuli e.g. You see a flower and you might think "pink" or "beautiful." You see the ocean and might think "vast" or "moving."
Some thoughts are more complex and are reactions to memories of external stimuli and combined with other memories e.g. "Imagine an ocean made up entirely of undulating flowers."
But what about an original thought? A thought that's not a reaction to any external event or object. Is it possible to form a thought from nothing?