Unlike signatures, spirits cannot be counteracted and do not fade away on their own. They are active processes which interact two-way with the matter they affect, changing it and also being changed by it. They do have some degree of free will, albeit it is extremely limited and are 99.9% determined by the ritual/repetitive action that created it.

Spirits gradually acquire other signatures, blending them into their own fabric, producing something different yet similar to their original form, acquiring slightly more free will each time they do so.

When a spirit reaches less than half (the percentages differ, and are damn near impossible to measure anyway) of its material the original signature type that created it, it becomes a soul.