It is easy to see how it measures physical strength. It is very objective. U lift it or u dont. The Bar makes no regard for your age, or gender, or stress level. It weighs the same regardless of who is lifting it.
Measuring desire may be a little less obvious, but when u have 5 reps to do and after the fourth u feel like youre going to die and are unsure if u can even do the 5th. Trying that 5th rep or not is the measure.
The bar develops strength much the same way as it measures it, however, to develop the strength u have to push the limit. Conveniently the bar is able to have weight added to it just slowly enough to keep making u stronger. And that weight must be added without fail in order to continue making progress.
Because the bar is great at measuring your desire, it is the perfect tool to develop the mindset required to accomplish goals and succeed at life. Because doing that 5th rep especially when u dont think u can, or dont want to is what makes the difference between the person who accomplishes goals and the one that won’t.