Excerpt from Howard Marks’ July 2003 Memo “The Most Important Thing”:
"One thing each market participant has to decide is whether he (or she) does or does not believe in the ability to see into the future: the “I know” school versus the “I don’t know” school. The ramifications of this decision are enormous.
If you know what lies ahead, you’ll feel free to invest aggressively, to concentrate positions in the assets you think will do best, and to actively time the market, moving in and out of asset classes as your opinion of their prospects waxes and wanes. If you feel the future isn’t knowable, on the other hand, you’ll invest defensively, acting to avoid losses rather than maximize gains, diversifying more thoroughly, and eschewing efforts at adroit timing.
Of course, I feel strongly that the latter course is the right one. I don’t think many people know more than the consensus about the future of economies and markets. I don’t think markets will ever cease to surprise, or thus that they can be timed. And I think avoiding losses is much more important than pursuing major gains if one is to achieve the absolute prerequisite for investment success: survival."
In security architecture terms, I differentiate Identity & Access Services which are designed to help enterprise achieve some business goals (and despite what a lot of people say about ROSI, these services have ROI attached to them from day one), but these are implicitly "I know" kind of service or least "I guess."
otoh, there are Defensive services like monitoring and logging, which implicitly say - "I don't know" how I am going to be attacked, how and where things will fail, but I need to build a margin of safety into the system to be able to react if and when they do.
The Security Triangle shows that depending on whether you have "I know" assumption or "I don't know" assumptions, you'll end up with a different looking architecture. Of course, its not a binary choice, you will have some of both, but there are always priorities and choices. The goal for security architects is to be clear about the choices, because if you are trying to know or accepting that you don't know, your security services delivery, measurements and processes will vary.
When you are building out monitoring services, your goal is to identify assets and event types with the goal of increasing visibility. This typically results in a people, process and technology like IRT that respond to catalysts and vectors that are often not known at the time the system is being built.
When you are building out Identity & Access services, you are assuming much more knowledge of subjects, objects, attributes, data and applications. This mapping typically manifests in architecture like Identity & Access Management systems, publishing and enforcing known known relationships.
In each of these cases, the toolsets are different, how you staff for them is different, the design and operations is totally different, but they get lumped under the title "security."