In Three Dialogues, Berkeley advances a theory of subjective idealism. Things only exist as experiences of individual consciousnesses. These things are static and themselves unthinking, as they are pure ideas. And they persist even when we are not looking at them because God is looking at them all the time.
Here's the thing, though---if esse est percipi, how do we exist? We aren't always perceiving ourselves. We sleep, for instance. And our actual selves are not just pure ideas. We are active and thinking. So we can't exist in God's mind in the same way that, for instance, a tree does.
If only unthinking substances can be perceived, and only that which is perceived can exist, how does anything exist to do the perceiving?