Say you want to achieve a certain output, like controlling your weight, for example. Then you would like to know what kind of actions and inputs are needed to get there. The output can be either a target, like losing 10 kg, could be controlling variability, like keeping your weight variation within certain boundaries, or maybe trying to understand where your weight will be given future changes in your environment. To make things easy, let's assume you want to lose 10 kg.
Now, when it comes to inputs, "eat less, move more" is too generic to get you there, but is a good place to start. The "eat less" part is the calorie intake factor which is a combination of how much, when, how and what you eat. Many things to keep track of and, in addition to that, there is also the way your body interacts with the food, which is not easily measurable. The "move more" part is even more complex and also harder to bring down to a number. So, you're facing a decent amount of complexity when it comes to mapping which inputs have what effects.
If you were to embark on measuring as much of the inputs as possible, estimating as accurately as you can the effects they have and then map that to explain different outputs, that would be quite a job. And if your passion is to figure that out, or you're doing body-building or whatever other reason which makes it worthwhile to invest that amount of resources in it, that's OK. Luckily, for the rest, there's the Pareto rule.
First, make peace with the idea that more often than not, knowing the last 20% will be of very little to no utility to your purpose. That is to say that 20% of "unexplained" outcome is an acceptable trade-off. Second, even if you were to do a fact-finding, deep dive type of analysis, it might be subject to specific environmental conditions for the period you're looking at or even your narrow perspective at the time you do it. So, even doing a 100% breakdown of the outcome might not be a true reflection of reality.
Well, what is true then? What would be a good reflection of reality? If we take the example of "laws of nature" or "laws of physics", these are equations that hold true no matter what. They provide a consistent way of explaining how nature behaves and why things are as they are. So, if we go back to our worldly problem of weight loss, we can infer that aiming for a consistent story of input and effects that explains 80% of the outcome should be a good reflection of reality.
To start with, we go on with measuring the amount of meals a day and the type (as in breakfast, lunch, dinner). This is pretty straightforward. We use an estimate for the amount of calories by meal and meal type and we get to the calorie intake number. This estimate can be a standard, generic one, or it can be based on historical data. Then we count the number of times we exercise and perhaps do a step count, that's also rather easy. We could add another layer of complexity by defining exercise intensity, as in 1, 2 or 3, assume a standard calorie burn for each type and then we got the calorie burn, the "move more" part. These two numbers + calories used when doing nothing and we have all the elements to calculate a calorie deficit or surplus. Then we take that deficit/surplus together with the outcome and pay attention to them over time.
By paying attention we start seeing nuances and learn from the patterns which are shown to us. Consistent follow-up will reveal whether some of our assumptions were wrong or whether the level of complexity is too shallow and we need to add some extra layers. We evaluate if our estimates and understanding of reality holds true in changing conditions. Maybe there is one other factor, say sleep, which is of such complexity that it would take an enormous amount of resources to understand its effects, and we figure out is the only one which matters in our weight variations. What if we would have invested our effort into measuring and mapping calories and none of that actually mattered?
A good reflection and understanding of reality is more about consistency than it is about accuracy or depth. Striving for consistency instead of the 100% may also serve as a mechanism for avoiding low-impact effort. Consistency reveals what holds true over time, and what holds true over time, matters.