Consciousness at its simplest is "sentience or awareness of internal or external existence". Despite centuries of analyses, definitions, explanations and debates by philosophers and scientists, consciousness remains puzzling and controversial, being "at once the most familiar and most mysterious aspect of our lives". Perhaps the only widely agreed notion about the topic is the intuition that it exists. Opinions differ about what exactly needs to be studied and explained as consciousness. Sometimes it is synonymous with 'the mind', other times just an aspect of mind. In the past it was one's "inner life", the world of introspection, of private thought, imagination and volition. Today, with modern research into the brain it often includes any kind of experience, cognition, feeling or perception. It may be 'awareness', or 'awareness of awareness', or self-awareness. There might be different levels or orders of consciousness, or different kinds of consciousness, or just one kind with different features. Other questions include whether only humans are conscious or all animals or even the whole universe. The disparate range of research, notions and speculations raises doubts whether the right questions are being asked.