Answer:
I would argue that Thomas' quote is accurate.
Explanation
The argument is simply about how to be authentic.
Thomas Merton's quote implies that it is counterproductive to focus on trying to be oneself or authentic.
His solution to this is for the individual to let go of the process consciously. In other words, the process of being oneself happens subconsciously. To consciously manage the process, it is possible to fall victim to drawing from the attitudes, mannerisms, etiquettes, etc because all our senses which are receptive input mechanisms are actively and intentionally involved in the process.
I would therefore argue in favour of the accuracy of Thomas' statement.
Suffice it to add that creating oneself is different from being oneself. The process of self-creation is intentional. To create oneself and still be authentic, one would need to intentionally evaluate, stimulate, amalgamate, accentuate and escalate one's strengths.
Given that no two individuals are equally gifted in the same measure, were each person to focus on creating their highest self, authenticity would be achieved. Even if two individuals were equally gifted, it is statistically improbable for both of them to give equal energy in maximizing their selves. Thus an inequality of persons would still exist in the end.
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