WHY aren't voltage sources open circuited and current sources short circuited? Thevenin's theorem (and even Superposition Theorem) states that we need to 'suppress' or 'nullify' the effect of all sources. We do this by short circuiting voltage sources and open circuiting current sources. WHY exactly can't we do this the other way round by open circuiting voltage sources and short circuiting current sources? What is wrong about doing that?