If Saturn is the expression of externally-imposed rules and order, and Uranus is about going beyond these boundaries to discover and create things that don’t obey and fit in the status quo, Neptune is formlessness itself. Neptune is where all boundaries, practicalities and materiality dissolve and everything is ethereal, boundless and nebulous. In astrology, the planet is associated with the subconscious, illusions, dreams, intuition, visions and haze.
The 3D, the things with form, have been completely transcended, and the entire human experience is a bullion of immaterial illusions, delusions, intuitive insights, visions, ideals, contemplation. Neptune merges everything with everything else. It erases the strict categorical divisions and underscores the unity and universality of the human experience.
The history of the planet's discovery supports the above associations. Neptune was found through mathematical prediction, not through empirical observation. Uranus was discovered because a scientist saw it. Neptune's story is different. Scientists observed Uranus and noticed that at some points in its orbit something interferes with its gravity. They speculated that somewhere in that area there was another planet influencing Uranus. Mathematical calculations were carried out and Neptune's location was estimated. Then the planet was discovered. In other words, Neptune was first imagined, then materialized. The law of attraction - first, you build an image in your head, then you manifest it. At first, this image is a lie, then it becomes reality - all reality starts out as a delusion.
Around the time that Neptune was discovered, Socialism emerged as an idealistic, altruistic vision about what society could be like and how the relationships between people can be arranged. Socialism is very Neptunian in nature. It started out as a very dreamy idea about people coming together as one (the ocean), doing things for each other and helping one another. The problem was that this vision was not realistic. There were always people with power and people without power, and the former always abused the latter. Eventually, that altruistic vision failed to materialize - it ended up as a delusion.
Neptune is associated with photography, film and acting. These arts focus on creating vivid illusions of entirely new worlds that look and feel real but are actually not. Everything in them is much more beautiful, loving and fair than the real world. There is a tendency to see things rosier than they are, so, again, illusions, dreams, delusions and lies. Visual arts are very immersive and dissolve the boundary between what is real and what isn't. Dissolving boundaries creates unions and makes everyone come together as a whole - the ocean.
The planet is named after the Roman God of fresh water, sea and everything associated with these such as naval power, maritime success, sea winds and storms.
The Neptunian tendency to dissolve the boundaries between real and imagined as also the reason why the planet is associated with medicine, drugs, alcohol, escapism, also mental health and hospitals. Films, just like drugs, help you escape reality. They favor a world that is much more altruistic than reality.
The planet itself has no solid surfaces, it is comprised almost entirely of liquids and gases. On it, nothing is fixed, nothing is solid. Interestingly, Neptune is warmer than Uranus despite the fact that it is further away from the Sun. Scientists have not been able to explain that and what is the planet's heating mechanism. It's weather patters is more active than Uranus's meaning that there are much more pronounced seasonal changes. This flexibility is similar to Pisces's fluctuating emotions.