Laudator Temporis Acti

Can you solve this riddle by Jonathan Swift?

Because I am by nature blind,
I wisely choose to walk behind;
However, to avoid disgrace,
I let no creature see my face.
My words are few, but spoke with sense;
And yet my speaking gives offence:
Or, if to whisper I presume,
The company will fly the room.
By all the world I am opprest:
And my oppression gives them rest.

Through me, though sore against my will,
Instructors every art instil.
By thousands I am sold and bought,
Who neither get nor lose a groat;
For none, alas! by me can gain,
But those who give me greatest pain.
Shall man presume to be my master,
Who's but my caterer and taster?
Yet, though I always have my will,
I'm but a mere depender still:
An humble hanger-on at best;
Of whom all people make a jest.

In me detractors seek to find
Two vices of a different kind;
I'm too profuse, some censurers cry,
And all I get, I let it fly;
While others give me many a curse,
Because too close I hold my purse.
But this I know, in either case,
They dare not charge me to my face.
'Tis true, indeed, sometimes I save,
Sometimes run out of all I have;
But, when the year is at an end,
Computing what I get and spend,
My goings-out, and comings-in,
I cannot find I lose or win;
And therefore all that know me say,
I justly keep the middle way.
I'm always by my betters led;
I last get up, and first a-bed;
Though, if I rise before my time,
The learn'd in sciences sublime
Consult the stars, and thence foretell
Good luck to those with whom I dwell.

The title of the poem is On The Posteriors, i.e. buttocks.

Note the pun in "My words are few, but spoke with sense," that is, with scents. "Through me, though sore against my will, / Instructors every art instil" refers to birching, and the "Two vices of a different kind" are diarrhea and constipation. But some parts of the poem are obscure to me, e.g. "By thousands I am sold and bought."