Eye position in texture space?

I’m currently working on a 3D fluid simulation. I’m following this paper from GPU gems; however i’m a bit confused about this part in the Rendering paragraph.

The ray direction is given by the vector from the eye to the entry point (both in texture space).

I already have the ray entry point, but i’m not sure how to get the eye position in texture space.

in “world-space”? then you need a transformation “from-world-space-to-texture-space” (the current location/orientation of the 3D-texture, i guess) and transform that “entry point” in that cordinate system