The cartoon depicts the simplest but most mysterious of situations: the mind in the act of perception. Consider the appearance of the cup in the mind of the observer. It is not the same thing as the cup itself, surely. It would be strange to say that the appearance is a physical thing of any kind, though it might correlate with physical events (a pattern of neural impulses or something). What is it?

The philosophical word for appearances is phenomena, and their study is called phenomenology. Phenomena include not just percepts, such as the cup, but hallucinations, dream-objects, and the objects of our memories, anticipations and wishes.

Notice that this terminology conflicts with ordinary and scientific usage, where “phenomenon” usually refers to something external.

Identifying appearance as a separate realm from the physical and mathematical goes back at least as far as Plato’s cave. In more recent times

Edmund Husserl launched phenomenology as a philosophical discipline of its own. Followers have included Martin Heidegger, Maurice Merleau-Ponty, Jean-Paul Sartre, Dagfinn Føllesdal, and many others.

Between these bookends the major figure is Immanuel Kant, for whom phenomena (Erscheinungen) were a principal concern. In his Critique of Pure Reason, the main thesis is that space, time, and the phenomenal objects that populate them are constructs of the mind imposed on sensation, rather than direct perceptions of external things. Experience is constructed by the mind using intuitions and categories that are built into mental architecture, and without that architecture, experience could not exist. He postulates an external world as a source of sensation, but concludes that our knowledge is restricted to the things of experience.

Here’s a simplified diagram of the process by which a phenomenal object comes into being, from Kant’s standpoint:

Let’s consider an example, wherein the object in question is a tree blowing in the wind. Over the time interval that we are considering, the intuition of space — a mental capability — creates a series of representations of a spatial object, the tree, in various configurations (say at times T1, T2, T3, and T4). So far, these are simply a set of different things, these representations, and there is no unity among them. It is the act of synthesis, which Kant calls an act of understanding rather than intuition, that glues these representations together into a single object: the wind — blown tree. Not only is it a single object, but it is registered in the mind as including causation for the motion of the tree arising from the wind. The category of causation is involved in the synthesis. The outcome of the entire process is not only a unified object, but the concept of how its behavior comes about. Kant says that the spatial structure, the unity of the tree-object, and the associated causality, are all due to the intrinsic nature of the mind — that we can only have experience by imposing these intuitions and categories on our sensations. Since experience can arise only in this way, the things of experience obey many laws. The source of these laws (“synthetic apriori propositions” in Kant’s lingo) can only be found in the structure of the mind and not the external world.

Let’s move forward by about one hundred years to Edmund Husserl.

Husserl’s main concern was also objects of experience and their structure. His teacher Franz Brentano had introduced the term “intentionality” into modern philosophy :

Every mental phenomenon includes something as object within itself, although they do not do so in the same way. In presentation, something is presented, in judgment something is affirmed or denied, in love loved, in hate hated, in desire desired and so on…. We can, therefore, define mental phenomena by saying that they are those phenomena which contain an object intentionally within themselves.

(Again philosophical jargon conflicts with common usage: philosophical intentionality has nothing much to do with the intention to do something).

Here we see a distinction being made between an intentional act, such as perception or anticipation (or affirmation, denial, etc), and the intentional content — the mental object perceived or anticipated. Intentional acts can be different but share their content. I can recall, perceive, or anticipate my favorite cup as full of coffee. It is the same coffee-filled cup (as phenomenon), but different intentional acts. Husserl’s concern was to analyze the structure of intentional acts and their contents.

In his later writings he used the terminology of “noesis” and “noema”:

Only the intentional act (noesis) takes place within the mind. When the act is taking place, it is directed towards its content: the noema. The noema (plural noemata) is not a physical object, but neither is it within mind. Rather it belongs to an abstract transpersonal domain(this is Dagfinn Føllesdal’s interpretation of the matter, anyway, and the one I find most comprehensible; there are several interpretations to choose from).

In order to grasp this view of things, it might be useful to consider the case where the intentional content is a mathematical object. If you and I bring to mind a sphere, there are not two separate topics of our thought, but one. The sphere belongs to the mathematical domain, which is common to all those who know mathematics. The out-and-out mathematical platonist will say that mathematical objects are just as real, and as external to ourselves, as physical objects. But what status of “reality” or “existence” to give to the mathematical world is a big and controversial topic. Its transpersonal nature, however, is hard to doubt. Husserl would say that the domain of cups and other ordinary things as intentional objects is also transpersonal.

We cannot conflate noemata with the physical objects themselves, for many reasons, but most simply because not every noema has any physical object to which it corresponds (consider dragons).

This idea of a realm of abstract variants of such things as cups will sound bizarre, most likely, at least to those who encounter it for the first time. Let me briefly describe a situation where this kind of abstract interpretation is the only one available.

Consider the computers that create the virtual worlds that we see in movies. I am not sure that Golum in the Lord of the Rings ever drinks from a cup, but if he did it would surely be a CGI (computer-generated-imagery) cup, which never had a physical existence. This is the situation:

There is no other comprehensible picture of the situation. It is not possible to make sense of the program without referring to the mathematics of 3D space generally, and of the shapes of cups in particular. The picture is correct in the sense that each of its parts do in fact correspond. One might say “but that mathematical cup doesn’t really exist” or for that matter “only the electrons and their travel exist, not those other levels”, but even so, the whole of the situation with all its parts and correspondence does fit into one mathematical structure, and I, for one, don’t see the point of calling some parts existent and others not.

I am not saying that the CGI computer’s operations are anything like those of the human mind, nor that the mathematical objects that are their content are the same as Husserl’s noemata — indeed they cannot be; a cup as experienced by a human cannot have its whole being in mathematics. I am only saying that the notion of human intentional content residing in an abstract realm has a counterpart in a something that we understand (unlike the mind): computation.

Back to Husserl. Having posited a structure for phenomena, the question is, how to go about investigating the structure? The challenge is to disentangle, in thought at least, the connection between appearances, and notions about the external world. Husserl introduced a technique called “bracketing” (or Epoché). It consists of putting aside considerations of physical existence — not denying nor affirming the existence of an outward object corresponding to the phenomenon under study, but excluding any reasoning that involves that outward object, even its existence one way or another. This is what is called bracketing the phenomenon. Consider my cup. I want to know how it works, as a phenomenon. What facts arise only from its intrinsic nature as an appearance? Husserl thinks he can derive many conclusions. Among the best known of his conclusions have to do with the “horizon”.

During the sequence of moments in which I have a cup in view, I see it as the same object, even though I may be looking at it from a series of positions. Not only that, but at each moment I have an expectation about the immediate future: that the cup will reveal its different sides as I move relative to it, whatever that movement may be. Although I might know my own coffee cup well, the same will not be true of other objects — e.g. a house or tree or sculpture only one side of which I have seen so far. Each phenomenal object has what Husserl calls a horizon. It is known to me only in part, but I hold the expectation of a future in which the object (or my perception of it) is extended various ways. This is like traveling over a horizon on the earth, except that it applies to every object at all times.

Here we see that the structure of a noema is rather complex. It includes a presentation (what I can see), and features that I ascribe to it. It also includes a notion of identity (this cup), which persists as time progresses, regardless of how the presentation and features may change. Husserl calls this identity the noematic core. So the account is: the cup as intentional content over time is a sequence of closely related noema, sharing a noematic core. At each moment, there is a horizon for the noema consisting of the possible noemata that lie in the future sharing the same core. Note that Kant’s picture was not entirely different: the presentations of noemata correspond roughly to Kant’s representations. The common noematic core is constructed via the synthesis of the understanding.

Note this account does not rely on any assumption about the existence of external objects corresponding to the noemata. The horizon-structure described above is an aspect of the phenomena themselves, or one might say, equivalently, of the mind.

Phenomenology does not end with Husserl, but he established the modern principles of the field. Heidegger, Husserl’s student, was a particularly significant figure in phenomenology’s further development, but he and other more recent contributors are beyond the scope of this brief note.