The scripture here gives us a key. Whatever has form is unreal. Finding the formless is the path to the real.Know that which has form to be unreal and the formless to be permanent. Through this spiritual instruction, you will escape the possibility of rebirth.
--- Astavakra Samhita (trans. Swami Nityaswarupananda)
So what is form? Form is something that has qualities: colors, sounds, scents, tastes, or feeling. Physical objects have form.
Throughout history and cultures, sages have often directed our attention to formless things as pointers to the truth. In Taoism, there is much talk about water. In Christianity, there is light. Tibetan Buddhists often talk about space.
In Hindu thought, the spectrum of existence is arranged from the absolute formless (nirguna Brahman) to absolute form or matter (prakriti). It is said that the formless gives rise to the form. This happens in a series of stages from the most subtle to the gross. The more subtle a thing is, the higher up on the spiritual hierarchy. Why? Because it is more like the highest.
The same goes for Taoism. The highest, the Tao, is without form. The lower end of creation is the 10,000 things. Creation goes from the Tao, to the one, to yin and yang, to the five elements, to all things. Returning to the Tao is ascending the ladder from form to formlessness.
In Christianity, the first thing that is created is light. Light on the first day, whereas the bodies of light (sun and moon) are created on the fourth day. The heaven and earth start out “formless”.
Accordingly, physical matter being mostly form is at the bottom. Thoughts are higher because they are less substantial. Awareness is higher because it even more formless than thought. Intuition is ranked higher than intellectual knowledge because it has less form.
Seen in this light, one can clearly see the pathway to return to the source.