Doubt until proof arrives
An important, widely held claim deserves doubt simply because no competent proof of it has yet been shown. Its popularity, age, or importance is not a reason to accept it; the burden lies on those who assert it.
Good experiments, bad explanations
People can be skilled at producing real results while being poor at explaining why those results happen. So separate what a practitioner shows you from the theory they wrap around it: like a trading voyage returning with gold alongside apes and peacocks, the useful findings arrive mixed with showy nonsense.
An element is what won't split
Call something an element only if it is perfectly unmixed: not made from any other substance, the ingredient that compounds are built from and finally break back down into. On this test, the fixed handful of traditional 'elements' fails, because the world's variety comes from smaller moving particles combining in many ways, the way a language needs many words to say complex things.