Length is considered a fundamental unit in physics because it cannot be defined in terms of other physical quantities. It's one of the building blocks that we use to define other, more complex quantities.

Here's why length is fundamental:

* Independent: Length is a fundamental property of space itself. It exists regardless of other physical quantities like mass, time, or temperature. We can't define it using any of those.

* Basis for other units: Length is used to define numerous other important units:

* Area: Length x Length

* Volume: Length x Length x Length

* Speed: Length/Time

* Density: Mass/Volume (which involves Length)

Think of it this way:

We can't describe how long something is without using length itself. It's a primary concept that we use to build our understanding of the physical world.

While it's true that we can measure length using other objects (like a ruler), this doesn't change the fact that length is a fundamental concept. The ruler simply provides us with a standard unit (like meters) to quantify that fundamental property.

Important Note:

While length is fundamental in the International System of Units (SI), in other systems, like the Planck units, it's possible to define all quantities in terms of fundamental constants like the speed of light and Planck's constant. However, even in these systems, length remains a crucial concept.