According to Google:
ob·serv·a·ble
əbˈzərvəb(ə)l/
adjective
adjective: observable
able to be noticed or perceived; discernible.
"observable differences"
synonyms: noticeable, visible, perceptible, perceivable, detectable, conspicuous, distinguishable, discernible, recognizable, evident, apparent, manifest, obvious, patent, palpable, overt, clear, distinct, plain, unmistakable
According to Wikipedia:
Observable
In physics, particularly in quantum physics, a system observable is a measurable operator, or gauge, where the property of the system state can be determined by some sequence of physical operations. For example, these operations might involve submitting the system to various electromagnetic fields and eventually reading a value off some gauge. In systems governed by classical mechanics, any experimentally observable value can be shown to be given by a real-valued function on the set of all possible system states.
Even by the Wikipedia definition, the existence of gravity is "observable". Try holding your arm out horizontally for a few minutes, and you will perceive all sorts of sensations caused by gravity interacting with your arm. How are the neural impulses caused by gravity interacting with sensory cells in your arm any different than the neural impulses generated by photons interacting with the sensory cells in your retina?
Stated differently, why would you consider sensory data captured by the retina to be an "observation", but not sensory data captured by other sensory organs? We don't "observe" anything directly; every sensory notion we have about the world around us is the product of some physical phenomenon stimulating sensory cells in our bodies, which stimulates neural activity, which is then interpreted by our brains. On what basis do you argue that only one human sensory apparatus can "observe" the world around us? On what grounds do you make that distinction?
