Commenting from the matter/energy definition of "exist" here.The only ascribable existence is to the medium containing the thought. Analogous to any information in a medium, thoughts are measurable because they are reflected in the arrangement of brain chemistry. We can of course recall information stored in our own brains, and as neuro-technology advances it's theoretically possible one day to objectively recall information stored in the brains of others.This is evidence of the existence of synapses, neurons, and neural transmitters, but the thought of an elephant or the number 3 doesn't have existence itself. I believe this resolves the apparent contradiction, but please correct me if needed. Thanks. I would like to discuss one more criterion for the definition of "exist".I have a Physics background, so something like thermodynamics is an important tool to me. Specifically, the Conservation of Mass and Energy is relevant here.If the combined mass and energy of a system is not affected by the presence or absence of a thing, then it can be said to not exist. This sounds a lot like "it exists if it's made of matter or energy", but I wanted to tie it into a law of thermodynamics and science, and get away from a semblance of arbitrariness.We could also consider entropy. To take a "thought" as an example, it reflects some ordered structure in our brains (a meme), and any order or arrangement is small but real amount of entropic energy. This is still consistent with the conservation Laws for two reasons:1. Two conflicting thoughts can't both be simultaneously valid, yet they both have energy. We can conclude that entropic energy doesn't provide validity to a thought.2. The gain in entropic energy as a meme is formed is countered by a loss of chemical energy (synapses fire, ATP consumed, etc). Thus, even with the creation of a new thought, the system's energy state is unchanged/conserved.I use "thoughts" as an example, but I suspect it's broadly applicable.