Hey poleflux, if you don't mind i would love to take part in this open conversation. My answer is quite long so please bear with me. Yes in a way or in a vague way a man knows that he/she will die, however this thought oscillates between conscious and subconscious. Sometimes we get an acute realization about the truth that we are going to die. Whether fortunate or unfortunate this realization doesn't sticks to us. I think that is the only reason we are able to maintain a balance and keep our sanity. So yes we human know that we are going to die and yet that realization doesn't drives out a certain zeal in us to cultivate and explore. I'm sorry if I'm generalising, of course I agree that their are many people among us( before us their were and ahead in time they will remain)but still a large mass despite knowing it (about death as inevitable) somewhere deep down in their sub-conscious are still ignorant in their action and thoughts. Only in some formidable moments they are brought face to face with such an intense feeling that curiosity arises and they question, but it doesn't lasts and they get back on their initial path, not always but often, Even this curiosity that arises among them on certain instances is out of fear, loathing and unhappiness. The actual sense of wonder about everything around us (which a child has) is gone, zeal to cultivate fades. So coming back to your question although it seems that human beings are the only creatures who know that they will die. My answer would be yes and yet they remain ignorant of it often. Now coming on the "only" part that I don't agree on. If you see animals, for that matter, any animal, I don't think it would be hard for you to arrive on a conclusion that they live in an organized way and serve a purpose. I would even go to an extent of saying that many aspects of their reasons and behaviour is driven from an innate sense of realization. From big animals to really miniscule one's we can see that they knowingly serve a purpose. Somethings are beyond our comprehension but that doesn't mean that its beyond any possibility. I'm sure you must be knowing of certain species that have a life span of one or few hours and in that time they do what is supposedly their purpose. They thrive to do that from their innate reason I must say. We study about animals, we predict their life cycle, even prove that, it would be really ignorant to say that they don't know what they are doing or their exist no thought behind their behaviour, rather they seem to be more enlightened about their purpose and thrive towards it with full sense of responsibility, even if they may not be aware of the inevitable death, they are aware of their purpose, on the other hand we human (only those who aren't living an ignorant life without any sense of wonder) take a plunge onto where our reason, logic, emotions takes us, I think that can be one purpose of human (not one human being but our whole species 'Humans") that we thrive on curiosity, cultivate, reason, question, contradict and always wonder at this stupendous and marvellous creation.