The myth is generally understood as a warning against self-love, Narcissus fell in love with his own reflection. According to McLuhan this is not quite the intention of the story. Narcissus was transfixed by his reflected image and so became numb to all other stimuli, a closed circuit.
All media are extensions of particular human aspects; the wheel is an extension of the foot, the lever an extension of the arm, clothing an extension of the skin, and so forth. Human invention is a response to need generated by discomfort; the wheel relieves the burden of moving objects, the lever the burden of lifting them, clothes keep us from being cold (or sunburned).
Any new invention is a greater or lesser shock to human relations. A neat illustration, from Rosa Luxemburg’s Accumulation of Capital:
In districts where natural economy formerly prevailed, the introduction of means of transport – railways, navigation, canals – is vital for the spreading of commodity economy… The triumphant march of commodity economy thus begins in most cases with magnificent constructions of modern transport, such as railway lines which cross primeval forests and tunnel through the mountains, telegraph wires which bridge the deserts, and ocean liners which call at the most outlying ports.
The latter chapters of the Accumulation of Capital are a meditation on the various media used to establish a commodity economy in various colonies, including the medium of ballistic weaponry. Colonialism is a rather sharp example but the point stands, changes in the medium of human existence require changes in the way people relate to each other.
On an individual level the shock of change leads to numbness, what might have been disturbing to your ancestors you have to take in your stride. Imagine, for example, your journey to work. You would never get there if you had to regard every single advert trying to catch your attention. This shutting down of the senses blinds us to the effect of various media. Back to the original example, ideology; we do not recognise mainstream ideology as such. Even so the supposedly non-ideological person is in fact the most ideological.
We only recognise a medium for what it is when it is either hybridised or superseded. An example from art is the journey from painted portrait to lithograph to photograph, to moving image, to synchronised sound, to Technicolor. Each invention cried out for the following one. As each medium was superseded it was transformed, the obvious example being after the rise of photography artists began painting concepts and feelings, rather than literal objects.
Another example: we now know that novels are in fact movie scripts. Every successful novel is touted to movie producers as a sure-fire hit (that or it’s cherished as an unfilmable novel). Movies are not novels, however. They almost never make the journey backwards. If anything movies are becoming role-playing computer games, judging by the number of spin-offs that have been made.
Relating this back to the point about ideology; we overcome our numbness to bourgeois ideology, see it for what it is, through its supersession (or, perhaps, hybridisation if we take reformism into account). This of course happens through practical action, class struggle, combined with the critical renovation of consciousness; the interaction, I would argue, between movement and party.