In act one, scene 2 of King Lear (lines 1-23), Edmund reconfirms his life's creed and its goals.
"Thou, nature, art my goddess": You, nature (natural selection, survival of the fittest, etc.) are my inspiration and leader.
"To thy law my services are bound": I am not bound by society's laws but by the laws of nature, (the law of the jungle).
"Wherefore should I stand in the plague of custom": Why should I be constrained by the limitations of society...
"and permit the curiosity of nations to deprive me": and allow its particular and arbitrary rules to hold me back...
"for that I am some twelve or fourteen moonshines lag of a brother?": just because I'm 12 or 14 months younger than my brother?
"Why 'bastard'? Wherefore 'base'?": Why am I called a 'bastard' (illegitimate)? Why am I considered 'inferior'?...
"When my dimensions are as well compact": considering that my body is as strong...
"my mind as generous and my shape as true as honest madam's issue?": my mind is as capable, and my looks are as good as those of a child of some respectable woman? (as opposed to that of a whore, like my mother).
"Why brand they us with 'base,' with 'baseness,' 'bastardy,' 'base,' 'base'?": Why do they stamp us with terms like 'inferior,' 'inferiority, 'illegitimate,' 'inferior,' 'inferior'?...
"who, in the lusty stealth of nature, take more composition and fierce quality than doth within a dull, stale, tired bed go to the creating a whole tribe of fops got 'tween asleep and wake?": us who, conceived in hot and hidden sexual passion, are created with more substance and strength than are a whole tribe of weaklings made by two people in bed who are half asleep?!
"Well then, legitimate Edgar, I must have your land.": Well then, my 'valid' brother Edgar - because you are one of those weaklings (created in the way I've described above), it is only fitting that I, the stronger and fitter brother, should have what is now considered yours.
"Our father's love is to the bastard Edmund as to th' legitimate": After all, our father loves me, the 'bastard,' as much as he loves you, the 'valid' one.
"Fine word, 'legitimate'": Fine word, 'valid.'
"Well, my legitimate, if this letter speed and my invention thrive, Edmund the base shall top th' legitimate.": Well, my valid one, if this letter (the fake one I've devised by forging your handwriting) works and my plan succeeds, I (the inferior one) will vanquish the 'valid.'
"I grow, I prosper.": I get stronger, I thrive.
"Now, gods, stand up for bastards!": Now, you, the powers-that-be, stand up for us 'bastards''!
Edmund, the son of a whore -- slighted, neglected and derided from birth -- has become as hard and tough in his heart and mind as he has in his body. As the play will show, he will do whatever it takes (deceive, betray, kill) to get what he feels is rightfully his.