The slipbox counters confirmation bias
Charles Darwin (1809–1882) wrote down arguments against his theories.
I had […] during many years followed a golden rule, namely, that whenever a published fact, a new observation or thought came across me, which was opposed to my general results, to make a memorandum of it without fail and at once; for I had found by experience that such facts and thoughts were far more apt to escape from the memory than favorable ones. Owing to this habit, very few objections were raised against my views, which I had not at least noticed and attempted to answer.
Darwin's technique is good but primarily mental. A slipbox helps make it natural to do this. It changes the writer's incentives from finding confirming facts (as when you write an article with an argument already in mind) to an indiscriminate gathering of any relevant information regardless of what argument it will support. Developing ideas bottom-up instead of top-down lets us focus on the most insightful ideas we encounter and welcome the most surprising turns of events.
We're still selective, but no longer filter by for/against but by relevant/irrelevant. The criterion is whether something adds to a discussion in the slipbox: "an addition as well as a contradiction, the questioning of a seemingly obvious idea as well as the differentation of an argument." In fact, "dis-confirming data becomes suddenly very attractive, because it opens up more possible connections", and "the experience of how one piece of information can change the whole perspective on a problem is exciting".