Summary: What's real and what's illusion? The word "measure" and the Sanscrit word, maya, have the same root ... and maya is the ancient word for "illusion" ... implying that what we can measure is the illusion.
An important systems thinking skill is "quantitative thinking."
This is one of the systems thinking skills described by the late Barry Richmond of High Performance Systems (now ISEE Systems). He argued we must extend our thinking to include seven different skills: dynamic thinking, system-as-cause thinking, forest thinking, operational thinking, closed-loop thinking, quantitative thinking, and scientific thinking
With quantitative thinking we accept that many important, even absolutely critical, variables (e.g., motivation, morale, burnout) in a system can't always be measured, but they can be quantitatively estimated. We accept that their impact on other, even hard, variables can be quantitatively estimated.
Otherwise we are limited to "measurement thinking," where we must ignore everything that can't be accurately measured in a sufficiently timely and efficient manner. This is a very limiting perspective.
When we ignore soft variables, such as those noted above, we make the assumption that the value of those variables is zero, which is the only value that we absolutely know is incorrect. We humans are part of the system, and therefore variables that affect our decisions and behaviors must be taken into account.
Joseph Jaworski, in Synchronicity, The Inner Path of Leadership (1996, p. 192) writes:
There's a wonderful section in [David] Bohm's book, Wholeness and the Implicate Order, where he talks about the root of the word 'measure.' "The western word 'measure' and the Sanskrit word maya have the same root. The word maya in Sanskrit is the most ancient word for 'illusion.' The prevailing philosophy of the East is that the immeasurable is the primary reality. In this view, the entire structure and order of forms that present themselves to us in ordinary perception and reason are regarded as a sort of veil, a veil that covers up the true reality, which cannot be perceived by the senses and of which nothing can be said or thought."