Like Mark said we actually know a whole lot about what causes cancer both from epidemiology and the molecular biology of cancer.We know which genes get damaged that lead to cancer and we know a lot about what compounds in the environment cause that damage. We know people with different versions of these genes have different relative risks of getting cancer.What is the biggest unknown is what will happen to you as an individual. Will you be the 1 in 1000 with your genes and your environmental exposure to get a specific cancer in a specific organ. We know that for each of the 1000 of you we can find there will be one cancer of that specific type, but which one is the one...that we do not know. But usually we know that people in a differnet group, much of this is environmental that you can control, like diet, smoking, etc. have a different rate... it may be people who smoke are 55 in 1000 and people who eat lots of fruits and veggies are 1 in 5,000So there is a fair amount of information one can make reasonably informed choices about which things one does is risky and which are not.Now some things we do know better than others, and sometimes something we think is true on deeper study is not, but that does not equate to no one has any idea at all.K.