So a plant has a soul, according to your Latin etymology, right?

Plant/plat - "to spread"... https://www.etymonline.com/word/plant

A plant implies the division of all (cause) into each (effect) plant aka the spreading of sameness (soul) by differentiation (spirit) into being (essence).

The plant you see represents a being (essence); the differentiation of each plant from one another represents breathing room (spirit/spiro - to breathe), and the unseen dividing animation represents the one and only (sole/soul).

According to it, it's hard to view the soul...

...because accordance with one another makes it harder to discern soul/sole (one and only) by oneself. Nature divides each native within from one another to allow each to discern self by analysis...others suggest accord to sow discord by synthesis.

Letting go of accord; prevents discord, which makes it easier for ONE to view the ONE and only sole/soul and what it does to each ONE within...which is division from ONE another aka making each essence diverse; unique; special; distinctive; notable; solitary (soul-itary) etc.

a plant as one that can never choose what to do

Only nature does (action); which allows each native within to redo (reaction)...each plant chooses how to react to being planted. Any gardener can tell you endless stories about plants choosing to be bitchy about growing as planted.

I do want the same from op on that one

That implies a difference (I) wanting sameness from another difference (op)...one can only find likeness this way. Sameness implies the nature each different native came into being within aka the soul/sole (one and only) for each solitary being within.

not supplying

Nature implies supply (need); native implies demand (want)...the struggle for life is about resisting the supplied temptation to demand more than offered by all from one another.

try to define

To define implies artificially "affixing" what nature "frees" from one another.