What is Matter
There is one thing which is common to Visible and Invisible Creation... and that is
Any visible or invisible substance which has Characteristic, Action and Cause of Existence, is MATTER.
For example, Air, which is a part of the Visible Creation, is always in motion and is ever dynamic. Dynamism of Air is its characteristic and motion is its action and the cause of its existence is Space. So, that is why, Air is Matter.
Now let us consider Soul, which is a part of the Invisible Creation.
Soul reacts in three different manners:
i) Shato Gun - The Soul acts like a mirror between the Psyche and Wisdom. Whatever there is in the Psyche gets reflected in a living being's Wisdom and vice versa. The Soul itself does not get affected but the Psyche and Wisdom do.
ii) Rajo Gun - In this state, the Soul is very reactive. It responds and reacts to all kinds of situations.
iii) Tamo Gun - This is just the opposite of Rajo Gun. The Soul is absolutely inactive. It does not react to any situation.
From the above, it is quite evident that Soul has characteristic and action.
The cause of existence of Soul is, the Cycle of Birth and Rebirth. Without Soul, the Cycle would not be complete and living organisms would cease to exist. Since living organisms are a part of Creation, it can also be said that the cause of existence of Soul is Creation itself.
That is why, Soul is Matter.