1. The existence of “rubber duck debugging”, and a whole bunch of studies on verbally explaining a concept indicate that language is essential for thought. In rubber duck debugging, programmers tell their problem to an object and this is beneficial in finding the solution. There are studies that show when we verbally teach material to someone we remember it better. Also the act of taking a test increases learning and memory, but why should this be if learning is secondary?

2. Everything we know about memory tells us that externality is essential for memorizing something. If there’s nothing visual, aural, or sensory then it is unlikely to be remembered. Language acts as an externality even as inner speech, meaning that thoughts can be said in language (ascribed onto the words) and remembered for short-term and long-term memory. A thought without externality seems more like a passing whim, unrooted in any more permanent mode of cognition and thus liable to be forgotten. I can imagine thinking in visuals, melodies, words, but if there is a kind of thought that isn’t occurring based off of these then it probably can’t be sophisticated.