Now admittedly yours truly was not born with a Mercury-Pluto trine--wish I had been but it's only a novile aspect for me (160/80/40 degrees), a 9th Harmonic aspect of 'destiny'. Curiously, destiny is also revealed in 6th Harmonic charts which is what I was investigating today and discovered that my H6 Mercury and Pluto are engaged in a trine. This sent me directly to Alan Oken's Complete Astrology to find out more, never having a reason to consider this trine as closely before.
Oken titles the Mercury-Pluto trine, "The Transcendental Mind" since it relates to higher consciousness and to a natural ability to "transcend certain set ways of thinking by viewing many levels of perception simultaneously." Consequently, this aspect is helpful in fields such as nuclear energy, education, advertising, or "other areas geared to molding of the mass mind" which sounds like propaganda to me. Of course, other aspects in the horoscope must agree for a Mercury-Pluto trine to express on these exalted levels (or at least not interfere!) Positive aspects to Saturn would help and a well-placed and/or well-aspected Sun and Moon would facilitate the beneficial energies of the trine.
Now as you know, Pluto is also the planet of The Collective Unconscious, described by Carl Jung as 'the intuitive history of the species since the beginning of time and that part of ourselves which combines instincts with conscious development' (paraphrasing). With the Mercury-Pluto trine, the consciously functioning mind blends well with the power embedded within The Collective Unconscious so that transformation of the higher thought processes can easily occur and creativity can express unencumbered.
Does all this sound suspiciously like transcendental meditation? Well, as a Christian I prefer the admonitions to 'still your mind' and 'think on good things' along with the power of prayer, but for those who prefer TM or are curious about the practice (Mercury-Pluto trine or not!), here's a one-minute-fifteen-second video on the difference between Transcendental Meditation and Mindfulness.
Yet while we're on transcendental subjects we must include researcher-philosopher-author-lecturer Terence McKenna whose presentations such as Last Thoughts - The Transcendental Object at the End of Time (part 27, a 15-minute segment from the full movie) remain fascinating! And intriguing is the natal horoscope of Terence McKenna if you're curious.
Born on November 16, 1946 at 7:25 am MST in Hotchkiss, Colorado (RR: AA), this tireless botanical explorer into shamanism and the realms of conscious and unconscious reality was born with deep-thinking Scorpio rising, Sun @23Scorpio38, Moon @1Virgo52, and Venus, Chiron, and Jupiter also in Scorpio. And the orb between his Mercury Rx rising in early Sagittarius and his 9th house Pluto Rx @13Leo22? Actually, it's 111 degrees 35 minutes which astrodatabank gives as a trine. Stretched a bit, perhaps, but with distant Pluto (and considering his lifelong studies) I'd consider the orb descriptive of a trine under the circumstances. Some of McKenna's quirkiness and interest in unusual or occult topics is seen by a Bowl shape that 'scoops up experiences' with progressive Uranus @20Gemini48 Rx leading all his planets. And of course, Mercury-ruled Gemini is the sign of The Mind.
To close, you'll find more details on Harmonic charts and why study them in a previous post containing a video presentation by master astrologer David Cochrane.
And speaking of botanical hallucinogens, here's a detail of my drawing of the Flower of Jamestown Colony, Jimson weed in bloom:
Terence Kemp McKenna R.I.P. November 16, 1946--April 3, 2000 with both transit Mars and Jupiter square natal Saturn in Leo conjunct Pluto Rx.