I am moved to muse on this topic, as I have been moved to several others on this blog by a production from my user-configurable dynamic textual projective surface, JanusNode, which recently produced the question: "Would you prefer to die of suicide with a group of others, or of thought cancer in a factory?" I had not previously heard the term 'thought cancer', and, it seems neither has anyone else. Googling it returns hits to 'throat cancer', and forcing the quote mostly gets sentences starting with "I thought cancer was...". The one hit for the term as JanusNode used it was an entry in the Urban Dictionary, which is not always a reliable source. It defines 'thought cancer' as "The adverse effects one endures mentally from over-thinking things; usually secrets, inside information, or just the side effects of an over-active imagination."
This is OK as far as it goes, but if cancer is a systemic error to which many independent biological systems are subject, I think there is a less metaphorical meaning for thought cancer: the unchecked growth of a thought. Many thoughts are cancerous in this sense: they are self-increasing. I had a friend when I was in university who was very sure that strangers were saying bad things about him when he passed them on the street. I could not understand why this otherwise-rational man would think this, until I did an experiment. I walked down Saint Laurent Street in Montreal, taking on exactly the same assumption. I was amazed at how easy it was to find evidence for the idea that people were saying bad things about me. I caught snippets of many conversations which involved people deriding...someone. Why not me? After all, it has been recognized since Aristotle's time (even though David Hume tried to take credit for it) that consistent contiguity in time and space looks like causality. If Event B occurs just after Event A often enough, we naturally assume that Event A caused Event B. If people are often using the word 'asshole' just after I pass them, it must be because my presence caused them to use it. My friend's delusion was just his brain doing business as usual. The problem was the more he believed it to be true, the more evidence he found for it, and the more he obsessed over it. Thought cancer.
This error of self-referentiality is really just a self-fulfilling prophecy, a well recognized mental error in Cognitive-Behavioral Psychotherapy. Many other psychological symptoms can have a similar underlying cause, most notably obsessive-compulsive symptoms. If I believe that I need to step into my house after a number of steps divisible by seven or terrible things will befall me, then I am going to get a lot of positive feedback for that belief. I always step into my house after a number of steps divisible by seven, and terrible things don't befall me! Phew. I dodge a bullet, again and again.
I believe the Web is now enabling thought cancer at a societal level. It does so has made because the Web has made it possible to tailor our news sources exactly to our beliefs, so we will only hear from people who think like we do. The result is the echo chamber of social media: we believe that everything thinks like we do because we have arranged things so that everyone we come into contact with does think like we do. This is the most dangerous thought cancer, because it leads to increasing isolation, which leads to increasing evidence that we are right in our beliefs. In a cancerous tumour, growing cancer begets cancer. Belief systems grow just like tumours, to the extent that we now have, where the two centre-right parties in the United States (i.e. the Republicans and Democrats, both of whom are far too right-wing to, e.g., have a chance of getting elected in my own native country of Canada) have somehow come to believe that they define incommensurably opposing views, rather than occupying a very narrow band of the political spectrum as they do.
Cancer treatment is very brutal, involving two main paths: excise the cells that are dividing too much (surgery), and then (usually) poison the patient to take her as close to death as possible without actually killing her (radiotherapy and chemotherapy) in order to kill all of the most rapidly-dividing cells: in this case cancer cells and hair cells (among others).
For the thought cancer that we now can now diagnosis– especially but not only in the United States– cutting out the tumour is hardly a viable treatment. America can't outlaw (excise) the Democrats and the Republicans (what else is there?), any more than it can outlaw the Internet that has allowed the two parties to to build increasingly stable, increasingly distinct, and ever-growing echo chambers.
The USA has now embarked on the only remaining option, the chemotherapy of its societal thought cancer. It is going to take itself as close to death as it can go without actually dying. Donald Trump is societal chemotherapy, the agent that will save the patient only by almost killing her. We don't need to admire Trump for this, anymore than we admire other poisons. But America's thought cancer has reached a near-terminal stage. The patient needs to take the poison if it is to survive, even knowing that the poison is going to make her vomit, moan for months, pull out all her hair, and degenerate into a long period of unproductive wasting. When it is all over, the uncontrolled growth of the cancer will be checked (through the destruction of the very narrow view of the political band that currently passes as reality due to the thought cancer) and the patient can return to her former glory.
Trump is not the only reason for the societal cancer; he is just a proximate cause. A good overview of the causal chain that led to Trump serving his poisonous role is given in Jonathan Rauch's (July 2016) Atlantic article, How American Politics Went Insane. I recommend it.
[On a related topic, see also my musings On The Narcissism of Small Differences.]