WORK IN PROGRESS. Please do not comment.

A logic device can be quickly defined, it's a snappy gadget. Formally it is apparatus such that all pertinent aspects of its operation obey the laws of Boolean algebra in a relevant sense. In clear terms, a logic device is something whose operation is dependent upon certain parameters, each of which can only adopt one of two distinct states. For example, a desk top lamp is logic device, its switch being either on or off, and its output is light or darkness. Perfectly logical operation does not mean something is a logic device. A lamp with a dimmer switch is no logic device, being off, partly on or fully on, and correspondingly being dark, dim or bright. That's the snappy part, things either are, or are not, and can only snap from one to another, with no half measures.

The majority of logic devices, by far, are now electrically operated. Of those, again by a wide majority, the devices are electronic in nature. But treating logic devices abstractly, their operation can be described without regard to the implementation technology, so that the theory is devoid of irrelevant detail. The theory is (hopefully) reasonably easy to follow without special knowledge, particularly of electrics/electronics.

Electronic logic devices can be very complex indeed, so an entire personal computer considered as one device can no longer be deemed of high complexity, being used as mere components in systems of yet greater complexity. Things as common as a mechanical door lock can be considered to be a logic device, keys being either accepted or rejected, entry permission being either granted or denied. The lock is more complex (due to the huge number of keys in existence) than the devices we shall consider, but they are not merely trivial. Using them, it is possible (at least in principle) to construct any other logic device, even a personal computer.

* Abstract devices and fundamental Gates
We use ALD as an abbreviation for "abstract logic device", and build our definition of an ALD in stages, the most difficult being first.

An ALD is a deterministic device, having a number of Boolean inputs, and one or more Boolean outputs (so as not to be entirely useless). Think of the inputs as controls or commands, imposed by the outside world, and the outputs as the exhibited behaviour or response of the device. A deterministic device has behaviour which is not the result of chance factors, but solely dependent upon past and current inputs. This precludes prediction of future inputs, and although prediction can be attempted, it can never be wholly accurate, as unexpected inputs can always possibly occur. (For example, modern processors do elaborate things, including branch prediction, so that typical programmes are executed faster, but this speed gain is lost when the prediction is incorrect.)

Many real world devices are accurately modelled by the ALD just defined. The main deficiency is that in the real world, a finite time is required for outputs to react to input changes. We shall accept this fact, supposing that ALD outputs achieve their intended state after a brief, but usually imperceptable delay.

We could now contemplate assembling ALDs to provide new behaviours, but instead merely state rules for doing so.