 A common theory-light approach deals with markers for consciousness. A marker is an indicator of something we cannot directly perceive, and in the case of consciousness we find markers by relying on our own phenomenological experience. For example, in his book Personhood, Ethics, and Animal Cognition (2012), Gary Varner relies on behavior and physiology of pain in order to argue that nociceptors connected to the brain, endogenous opioids, responsiveness to analgesics, and appropriate pain behavior are sufficient indicators of consciousness. Scientific data current at the time suggested to Varner that all vertebrates experience pain, but among the invertebrates, the current science only suggested pain in cephalopods (such as octopus, squid, and cuttlefish). More recently, Tye’s Tense Bees and Shell-Shocked Crabs: Are Animals Conscious? (2017) suggests we use Newton’s Principle (“from similar effects we can infer similar causes”) and the premise that certain human behaviors are caused by consciousness to infer that certain animal behaviors are caused by conscious experience; this approach finds consciousness in crabs, bees, and fish, as well as mammals, birds, and reptiles.