Magic and 'science' do not oppose, repel or interfere with each other. People who understand some basic sciences are better and more efficient magicians.
This article deals with the specifics of how magical processes work in all Muttiverse worlds. It does not deal with how much individual cultures understand about magic at any time in their development.
Magic in muttiverses is an omnipresent, fundamental background stuff. In some respects it is like aether [*].
Magic is not directly detectable, but it is nevertheless manipulable by some individuals and methods. It's like a catalyst - it isn't changed or used up in magic-workings or spells, but its presence makes processes possible which otherwise would not be (they would require vast amounts of energy). The energy cost for magic-catalysed processes is reduced, but the energy must be provided by the practitioner or some other power source, not the magic itself.
But if it's the same everywhere and can't be directly detected, how do we know magic's really there? Is there any reason to assume its existence? Perhaps there's another explanation for the reduced energy cost? Well, actually, magic isn't the same everywhere. In a very few locations, it is 'thinner' (processes run more slowly) or 'concentrated' (processes run more easily/quickly). These are large areas, on the order of solar systems or stellar neighbourhoods.
There are entire worlds in the Muttiverse with no (or very little) magic. This is why.
A caster casts spells; a worker performs workings. "Cast" can be an objectless verb, as "work" is in this context.
References to 'energy', unless otherwise stated, are talking about biological energy (what you get from food) and not mental or 'psychic' energy. In this system psychic energy doesn't exist, and mental energy is an idiom for mental effort or concentration.
An active magical process has a location in space. The location is wherever the process is producing its effect. If that's not inside or next to the magic-user, there will also be visible signs of magic manipulation on the magic-user, though no 'string' connecting them.
A magical process has a physical size. A more complicated process is physically bigger.
Workings are 'performed', not 'worked'. People who perform workings are 'magic-workers'.
Performing a working is comparatively slow, labour-intensive and flexible. They require more energy and concentration than a spell. Magic-workers are limited mainly by their personal capabilities and imagination.
At its most basic, a magic working is the manipulation of a force (a push, a pull or a twist - eg gravity, electromagnetism, friction).
However, most magic-workers (whether or not they understand post-Newtonian science) do not think of it in this abstract, theoretical way. They will approach it on a more instinctive, intuitive level.
Most magic-workers use individual mental techniques (see Ways of manipulating magic) depending on how they were taught, the particular things they're good at, the way in which they sense magical processes (see Magic senses) and their understanding of the principles involved. There are generally benefits and drawbacks to any such technique. Some may use it in a similar way to telekinesis; people with a material focus (see that section) can work in ways resembling photokinesis, thermokinesis and so on. Of course, there is a lot more to magic-working than moving things around; it is possible to change temperatures, shape light, create sound, manipulate friction, have knock-down whizz-bang magical firefights and even, for people with the talent, affect the basic principles of life enough to make things heal or grow.
Spells are 'cast'. People who cast spells are 'spellcasters'.
Casting a spell is comparatively fast. They require less energy than workings. A single spell always does the same thing when it is cast. Spells can be written down and taught to other people.
Spells can do the same things workings can, but the caster can't change what they do, only set them going (and stop them, if they don't stop by themselves).
All spells are cast on a location (or object or creature). Some spells need to be given specific instructions at the time of casting, like "for three seconds". Some don't need any instructions, like a simple firelighting spell. Some don't accept instructions when they really should, and are consequently pretty inflexible or useless. As this implies, a spell is only as good as its creator.
Turning a working into a spell is a separate, and rarer, skill. People who can create new spells like this are 'spellwriters', occasionally called 'mages' (though note that various cultures use the term 'mage' to mean different things).
Unmaking a spell to tinker with it is possible but time-consuming and painfully complicated. Not all spell-writers possess the aptitude.
Many magical processes, particularly spells, do have perceivable effects associated with them. These are sometimes accidental if the magic-user is sloppy or unskilled, such that some light, colour, noise etc 'spills over'. Often, though, magical effects are deliberate. The reasons for them are various: they may be intended as a courtesy, a warning sign, a 'trademark' for a particular spellwriter (one famous Shaded mage caused all her spells to be accompanied by the smell of fresh mint) or simple showing-off. There are occasional fads for particular magic effects, such as flashy lightning bolts, sparks and one short-lived, generally unmourned craze for a particularly annoying "woccawoccawocca" noise.
An individual magic-user's unconscious visible magical effects (the accidental sort) tend to look the same - the same colour light, and so on. This has sparked a few pseudo-scientific beliefs, including chromatic theory.
Every magic-user has a different way of sensing and controlling magical processes. Some magic-users 'see' processes, some 'hear' them, some experience them as heat or texture or rhythm. It can depend on other talents the magic-user possesses: natural musical ability, for example, means an aural bias is likely. Most people are visually biased. There are quite a few aurals, and some people who 'feel' it as something like static, or an attractive gravitational force, or just describe it as a general sensation of 'power', etc.
Some of these obviously allow the sensor to sense more fine detail than others. All magical senses become more sophisticated with experience.
While typical magic-users can sense that there is a magic effect and usually pinpoint where, that doesn't mean they can tell what it is doing. If it's something obvious (like flying around or sudden combustion) anyone can put two and two together. If the magic-user is familiar with the style involved - if it's a particular person or school de knows well - it will be easier to tell what is going on.
A Muttiverse magician might miss continuous low-level effects, or only notice them subconsciously. If there's something with an inactive enchantment on it, not currently doing anything, the magician's magic sense may not pick it up at all.
Illusions are seen as they are designed to be seen, but will also be obviously magical.
Magical effects can sometimes be masked from magical senses, given enough know-how.
If you happen to work magic by whistling, not everything you whistle will be a working. Just because whistling is how you activate the magic-manipulating part of your brain, that doesn't mean you might not just be whistling a tune for the fun of it. But then again, if you're an absent-minded sort, you could most definitely work something by accident.
Your material focus might be gases if you're a particular whizz with weather patterns and manipulating air pressures, or metals if you find you've got a talent for magically-aided blacksmithing and finding stress points... and so on.
Though many have a material focus, most magic-users are not material-minded (see next section).
Full article: Material-mindedness
Magic processes can occur without conscious effort from an intelligent being. Some magic-catalysed effects may conceivably happen by chance, but usually such magic processes are deliberately but indirectly caused.
It is possible to embed spells into objects. (Working requires conscious prompting from start to finish, so workings by definition cannot be embedded.)
Spells cannot be put into preexisting objects, but must be added while the materials are being shaped or formed. The range of materials that can take spells is extremely limited.
The energy cost of the spell must be provided by an outside source (magician, battery, solar cell...). It cannot be 'embedded' using the same method by which the spell is stored.
Shade has relatively many magic-users of widely varying skill and technique, and a tradition of research and teaching.
Main article: see Shade's Magic.
Offwhite City have very little magic capability and use it only for ceremonial purposes. Spells are outlawed.
Main article: see Magic (Instar)
If something is causing a continuous low-level effect, and the magician did not witness it being 'turned on', the magician may overlook it or only notice it subconsciously. If there's something with an enchantment on it but it isn't doing anything (for example, something that needs a specific trigger before it activates) the magician may not notice it at all. Even so, it's helpful if you mention them to the writer.
War magic-using community does not understand the nature of magic-as-catalyst and individual magicians would not use all this jargon comfortably or understand the science in depth.
How you interpret magic depends on your magic sense. The most common sense is visual, but synaesthetic magic senses (ones that seem like a mixture of two or more senses) are extremely common. Thus, for example, you might have a combination of visual and auditory senses and detect magic traces as a mixture of monochrome blobs and notes of varying pitch. Even two people's visual senses can also differ; you might differentiate a magic effect by its 'texture', its colour, its opacity or a mixture of those, and an auditory person might hear musical notes, white noise, humming and whistling or even syllables that sound like speech.
The nature of your magic sense makes a practical difference if it's a sense you can turn off - for example, visual senses don't work if your eyes are closed. The nature of your sense also makes a difference to its sensitivity: hearing and sight can give far more information than a 'gravity' or 'balance' sense. (But the latter two can be fun to write.)
Choose a magic sense that makes, er, sense for your character. A visual artist is likely to be visually biased; a werewolf, if a world contains such things, would be likely to use des nose.
Sort out your method of magic-manipulation first - does your character sing it, dance it, clap it or use some mental technique? Does de have a particular material focus? All of this will determine des understanding of what magic is. For example, imagine being an uneducated magic-worker with a material focus for gases; perhaps you would think of it as "my secret game with the wind". Or, alternatively, "I can blow out a candle from across the room, which really impresses girls". Or "I'll tell you a secret. I can make things happen. When I got really angry with that man, I wanted to squeeze him... next thing I knew, he was on the floor - he had stopped breathing for most of a minute."
Casting: this requires memory and a source of spells - someone needs to have taught you them, or you found them in a book, etc. Spells are not 'forgotten' once cast, but can be forgotten from disuse or bad memory. They can be partially forgotten or misremembered (they will probably not work at all). The basics of casting boil down to 'twist the magic-interacting part of your mind in this sequence', but your character will do this indirectly and symbolically by des chosen method. Material focus, if any, has little effect on casting.
Working: this is improvised and can be methodical or instinctive. Practising a certain sequence of working will help a character get faster, but it will never be as fast or effortless as a spell. Your character's material focus, if any, has a large effect on working, in that manipulating that material will come more naturally and seem to be less tiring (though any actual difference in energy outlay is probably insignificant).
Choose a method of manipulating magic. You cannot do this 'just by thinking about it'; there has to be some kind of physical action (gestures, movements, various ways of using the voice; plain speech along the lines of "make that thing explode" will not work. The manipulation must be symbolic).
* [Wikipedia] describes aether thus:
Like aether, Muttiverse magic has no measurable qualities and is incapable of change. Unlike aether it has no effect on mass and so no bearing on the movement of planets. Unlike aether, magic (merely by its presence) makes it possible to find 'shortcuts' to cause some physical processes to require less energy than they otherwise would.
Like aether it has variances in density, but unlike aether these aren't related to physical matter - certain regions of the universe, much larger than planets or even solar systems, just naturally have denser or less dense magic. Much more localised effects could be possible, but we haven't yet come across any.