If I were to give modern definitions to Aristotle's concepts of substance and form, I would say "substance" is any particle that had mass, and "form" is the state of that matter which defines how it interacts with other matter. If we look at the structure of an atom, the nucleus is composed of protons and neutrons, around which electrons orbit. Protons and neutrons is what provides the most mass of the atom - this is what was previously known in philosophy as "substance." The electrons which orbit the nucleus have very little mass, but are constantly changing in state: this is the "form" of the substance. The electrons are what provide form to the substance of the atom: they define how the atom bonds with other atoms, and they are constantly changing in state. If we were to look at the substance of water, its state of liquid, steam or ice is defined by the state of its electrons. Protons and neutrons, for the most part, are static and just provide the weight.
Recently, scientists finally confirmed the existence of the Higgs-Boson particle, and this is apparently what provides mass to all other particles. As I understand it, there is a field that permeates all of space, and the more each particle interacts with that field of space, the more "mass" it has. Imagine objects travelling through water: the more resistance the shape of the object has to the surrounding water, the slower it travels. In the subatomic world, this resistance to the field of space is "mass." In one of his visions Swedenborg had a conversation with Newton, who stated that the biggest mistake he made in physics was that he conceived of space as "nothing." Space is not nothing, it is a field - and modern physics has confirmed this. Space and time are created things.
Substance and form, at a subatomic level, is defined by how a particle interacts with space and time. The more a particle interacts with space, the more mass it has. The more a particle moves through space, the more it provides "form." Movement through space, or changes in state in space, is what gives us our concept of "time." Without a change in state or movement, there is no time. Particles which move the fastest - photons and electrons - are those which best provide the form of substance or matter. Photons have no mass - photons provide light, and it is light which provides our concept of form for all matter.
Substance and form, or space and time, are the two principles which underly all that exists in the universe. In the spiritual world, Swedenborg saw that the realms of heaven and hell exist outside of space and time. It is this world which existed before space and time were created in the physical universe. In the spiritual world, instead of space, there is love. The closer one feels to another soul, the closer one appears to that person. Thus spiritual societies are formed according to one's love or interests. "Birds of a feather will flock together." You will be with the one you love the most in heaven. In the spiritual world, instead of time, there are changes of state. In the physical world, the reality that we know as space and time corresponds to love and truth in the spiritual world. Love and truth corresponds to man's will and understanding. This spiritual world is described in more detail in Swedenborg's work, Heaven and Hell.
In the spiritual world, instead of physical substance and form, and instead of space and time, there is nothing but love and truth. Anything that appears in the spiritual world has a correspondence to love and truth. Moreover, love and truth are not simply abstract ideas: love and truth are spiritual substances, a higher level of existence which does not interact with our own five physical senses. And this is the subject of the book, Angelic Wisdom concerning Divine Love and Wisdom. Emotions, feelings, intuition - all of these which we experience internally have a true spiritual reality:
"...the substances and forms which are love and wisdom are not visible to the eyes like the organs of the external senses. And yet no one can deny that those things of wisdom and love which are called thoughts, perceptions, and affections, are substances, and forms, and that they are not volatile entities flowing from nothing, or abstracted from real and actual substance and form, which are subjects. For in the brain there are innumerable substances and forms in which resides all interior sense which has relation to the understanding and the will." (Divine Love and Wisdom, n. 42)The way Swedenborg attempts to demonstrate this is that he explains that our knowledge of substance and form comes from how those external objects affects one of our five external senses. Exterior substances and form thus correspond to more interior spiritual substances and forms in the brain: everything that we experience through the senses will trigger a corresponding affection or thought. However I have sought to explain it in a different way according to how substance and form are related to space and time, and how space and time correspond to love and truth in the spiritual world.
If love and truth are truly spiritual substances beyond the physical, then what is the ultimate form in which love and truth appear? According to Swedenborg, the ultimate form of love and truth is the human form. The will (love) and understanding (truth) are made one in action, or in being useful. And the form that is the most useful or utilitarian is the human form. Thus, "all of heaven conspires to the human form" (Heavenly Arcana, n. 5110.3), and, "every society in like manner forms a man; for the image of the whole heaven flows into the societies, and causes them to be like; and not only into the societies, but also into the individuals in the society, whence each individual has a human form, for every one in an angelic society is a heaven in least form" (Heavenly Arcana, n. 6605). Also, "the angels themselves appear in a human form in the heavens altogether according to the truths which pertain to them in good, with a beauty and brightness according to the quality of the good from truths" (Heavenly Arcana, n. 8988.3). Man, having a human form, is heaven in a microcosm.
As the ultimate form of love and truth is the human form, the foremost insight that Swedenborg makes is that God Himself is Divine Love and Divine Wisdom: He is thus Substance and Form itself. And as He is the origin of all things of substance and form, He indeed is the very-most Man, the God-Man, and appears to angels in human form. Thus it is true that God made man in his own image. The ancients, who worshipped God as Man, did not anthropomorphize Him, they knew the truth. The opposite is true: God anthropomorphized man. God is not an abstract concept: He is the One Being who has a Human form, from whom all things exist. This is the reason He could make an appearance on our earth, incarnated in the human form of Jesus Christ.