The existence of an idea results in that idea being perceived. We perceive only our own sensations or ideas. External objects are perceived through our senses, but we only for sure perceive our own sensations or ideas. If an object exists it must be perceived by myself or some other perceiver. There are endless ideas or objects of knowledge. In order for us to imagine or remember them, there is something that perceives them, which is the mind, spirit, or soul. In terms of existing, it is applied to sensible things. Berkeley uses the table as an example of how existing is used in this context. If I am now using a table to write this, it exists; I can touch it and see it before me. If I am no longer using a table it existed. So by senses we can perceive the table to exist. I can't see or feel anything without the actual sensation of that thing, so Berkeley states "it is impossible for me to conceive in my thoughts any sensible thing or object distinct from the sensation or perception of it (Berkeley, Classics of Phil). If I can't conceive it, or "that of any other created spirit  (Berkeley, Classics of Phil), it must not exist at all or subsist in the mind of an external spirit. There are primary and secondary qualities. The primary qualities are patterns or images of things that exist without the mind (matter). These things are set and stone without us having to even think about them. The secondary qualities do not exist without the mind, so they are sensations that exist in the mind alone. Number is a primary quality but it is different because it is hugely dependent on the mind (understanding). But how do we know all of this? We must know it by sense or by reason. Since we only perceive or our own senses, reason allows us to perceive external things. So even if you imagine trees or books you are doing so by forming ideas through imagination. The cause of ideas is active substance or spirit. The spirit is "one simple, undivided, act