Is Upekṣā (equanimity) a feeling or simply numbness?

Feelings are like clouds in the sky. They are seen, but we don't build our houses on them. Clouds come and go, sometimes bringing rain and wind and sometimes not.

SN36.19:5.3: In one explanation I’ve spoken of two feelings. In another explanation I’ve spoken of three feelings, or five, six, eighteen, thirty-six, or a hundred and eight feelings.

Clouds are beautiful and mesmerizing. Liking feelings and clouds, we might even chase them:

SN32.13-52:1.11: ‘If only, when my body breaks up, after death, I would be reborn in the company of the gods of rainy clouds!’
SN32.13-52:1.12: They give food …
SN32.13-52:1.13: a lamp.
SN32.13-52:1.14: When their body breaks up, after death, they’re reborn in the company of the gods of rainy clouds.
SN32.13-52:1.15: This is the cause, this is the reason why someone, when their body breaks up, after death, is reborn in the company of the gods of rainy clouds.”

And if cloud-chasing proves unfruitful, we might then stubbornly ignore clouds, trying hard to be numb to clouds, stopping or repressing any kind of feelings that may arise about clouds. But if we become numb to clouds, we'll likely be surprised by storms on some days and sunburned on other days.

However, if we watch clouds carefully, with equanimity, we might gain insight and be better served by knowing when to plant, when to harvest and when to seek shelter. Watching clouds very carefully, we gain insight into their coming and going. That very insight leads to a peaceful pleasant feeling that is finer than chasing clouds or feelings.

SN36.19:6.12: Because there is another pleasure that is finer than that.
SN36.19:7.1: And what is that pleasure?
SN36.19:7.2: It’s when a mendicant, quite secluded from sensual pleasures, secluded from unskillful qualities, enters and remains in the first absorption, which has the rapture and bliss born of seclusion, while placing the mind and keeping it connected.

Equanimity allows us to observe carefully. And what do we observe carefully? Well, the second of the four mindful meditations is actually the meditation on feelings. We observe feelings rid of desire and aversion.

MN118:25.1: Whenever a mendicant practices breathing while experiencing rapture,
MN118:25.2: or experiencing bliss,
MN118:25.3: or experiencing these emotions,
MN118:25.4: or stilling these emotions—
MN118:25.5: at that time they meditate observing an aspect of feelings—keen, aware, and mindful, rid of desire and aversion for the world.
MN118:25.6: For I say that close attention to the in-breaths and out-breaths is an aspect of feelings.

In other words, we observe feelings undisturbed by them. We observe, "this is a pleasant feeling." We observe, "this is a painful feeling." We observe, "this is a neutral feeling."

If we watch clouds with equanimity, we might see the ocean that gives rise to the clouds. We might see the mountains that catch the rain and channel it to the sea via streams and lakes. With equanimity we might see beyond the clouds, beyond the feelings:

AN10.61:4.1: It’s like when the rain pours down on a mountain top, and the water flows downhill to fill the hollows, crevices, and creeks. As they become full, they fill up the pools. The pools fill up the lakes, the lakes fill up the streams, and the streams fill up the rivers. And as the rivers become full, they fill up the ocean.

If we define numbness to sensory input as Upekṣā, we should be able to reach Upekṣā by severing our sensory inputs. But it does not happen that way.

The problem is not in the sensory inputs. The problem is that we evaluate the sensory inputs, assign a value to it, then start building attachments or resistance towards it, based on the value assigned by ourselves. As we chase the illusion, we are not reacting to the actual sensory input. Instead, we build our own version of the world within our mind, assign values to it, and form attachments to them or build resistance and fight with them. All of this is done for one purpose: to prove to oneself that there is 'oneself' and a 'world outside'. The reality is that there is just a process of sensory input interpretations, the thought of that interpretation and oneself are not two things, its one and the same. We fool ourselves (asmimana) building a world just to prove our-self that there is 'me' because 'I' was able to recognize 'you'.

In Arhath state, this process does not take place, so there is no attachment nor resistance as the need to prove that there is 'me' and 'there exists a world according to me' is no more.

Keep in mind that Upekṣā (equanimity) is an effect of understanding the true nature of the world. If you try to be numb towards the sensory inputs you are making Upekṣā a cause, not an effect. The difference between these two are much like an actor playing the role of Abram Lincoln, compared to the real Abram Lincoln. One is genuine, while the other is just a pretender. Rather than, trying to reach Upekṣā forcibly, try to look into the process that takes place within your mind when those sensory inputs hit you. It would allow you to escape from this dilemma.