Idiom, phrase, or word for "no clear division"

Example: "There is no clear division between a liquid and a solid. The soggy soil we are standing on is a case in point. So is the yogurt you're having."

What's an idiom, phrase, or word for "no clear division"?

I thought of "fine line", but it doesn't seem to be appropriate in this case; a "fine line" is usually used in a cautionary way, with the second object being unfavorable in some way - for example, "There is a fine line between being funny and being immature."

Solution 1:

Probably indistinct may suit your context:

Not clearly or sharply delineated: an indistinct pattern; indistinct shapes in the gloom.

From the Free Dictionary

• An indistinct mass of liquid and solid.

Solution 2:

One idiomatic expression for this is much of a muchness

(idiomatic) Of two or more things, having little difference of any significance between them.

Source

Solution 3:

Try continuum.

Continuum noun A continuous sequence in which adjacent elements are not perceptibly different from each other, but the extremes are quite distinct: a continuum of special educational needs - ODO

Here's an example from MW:

His motives for volunteering lie somewhere on the continuum between charitable and self-serving.

In your example, you could say, "Liquids and solids are part of a continuum." or following the form of your example more directly, "There is a continuum between a liquid and a solid."