Depending on meaning,
mental
automatic
pedal
by hand, manual
manual
In general, "mechanical" is not the opposite of "authentic." The opposite of mechanical is manual. The opposite of authentic is fake.
The opposite of mechanical (by machine) could be biological or natural.The opposite of mechanical (perfunctory, without thought) could be conscious, considered, or thoughtful.Where mechanical has the connotation automatic, the opposite would be manual.
Position one person on the opposite side of the patient
Check the DMV manual.
Base models are manual windows and locks. LSI models are opposite of base models.
If a lawn mower is powered by gas, that is the opposite of electric. If a girl you meet is one day electric and the next day the opposite she is listless or aloof or worse, boring. If it is Bob Dylan who has gone electric the opposite was acoustic. If Ed Begley Jr. doesn't drive a gas guzzling SUV it is because he drives electric. If you are cutting edge with your power then the opposite of electric is either solar or nuclear depending on your own tastes. AC/DC!
Go to the download section of the vestal website http://www.vestalwatch.com/#/Downloads/ You can download the manual for G5 G7 and G9 watches. Note on my G5 the EL/SET and RESET buttons are the opposite way to those shown in the instruction manual You can also request a PDF of the instruction manual by emailing info@vestalwatch.com
The opposite (antonym) of opposite is same.
A manual filing system is filing documents using paper and cabinets. The documents are filed within a folder and then stored, by some means of categorization or another, in a cabinet for later retrieval. This is the opposite of storing documents within folders on your computer's hard drive. In the computer world the documents are stored in a digital filing system, as opposed to the physical filing system in the manual way.
opposite of opposite to opposite from