In ancient Greece, philosophy was a practical skill allied to living the good life. It took the arrival of tenured academics to turn the discipline into at worst a deliberate irrelevance and at best a fount of knowledge divorced from spiritual growth.

Today anyone searching for the meaning of life is likely to find few pointers in professional journals. Self-help books and medication are offered as crutches instead.

For author and academic Stephen J Costello, philosophy needs to reclaim its therapeutic roots – but also our medicalised approach to unhappiness needs to change: in short, we need less Prozac and more Plato. As director of the Viktor Frankl Institute of Ireland, Costello is an advocate and tutor of "logotherapy", which blends counselling methods with a personal quest for purpose. The practice was designed by Frankl in response to his imprisonment in Nazi concentration camps during the Holocaust, as documented in his influential book Man's Search for Meaning. Frankl was fond of quoting Nietzsche: "He who has a why to live can bear with almost any how", and while he used some technical jargon to explain logotherapy the core principles are very simple.

In the final Unthinkable in the current series, Costello invokes Frankl to provide today's idea: "Meaning correlates to mental health. If you don't have meaning you won't have mental health."

Is logotherapy for everyone or just those in need?

"Frankl construes the human person in three dimensions: body, mind and spirit – or soma, psyche and noös. Even if body and mind are broken there is what he calls the defiant power of the human spirit and the spirit can never be sick; this is his psychiatric credo.

“So logotherapy wouldn’t be just for people who have problems – and who doesn’t, of course – but it would be for people who would want to reach a stage in their lives of fullness or flourishing when previously they had been floundering.”

How does one discover the meaning of one’s life?

“Frankl would distinguish between the meaning of the moment, which is unique, variable, changing, depending on the singular situation, and the meaning of life itself. The meaning of the moment now is for me to talk clearly to you, and to listen to your questions. When you leave that meaning will be replaced.

“Now if you raise the question of ‘the meaning of life’ Frankl would say this is like asking a chess player what is the best move in the game; it will always be dictated by context.

“The meaning of life is a question for religion, the meanings in life a question for logotherapy. But broadly speaking Frankl would highlight there are three main ways we find meaning in our life: creatively (the things we give to the world), experientially (the things we receive from the world), and attitudinally (how we respond to blows of fate).”

Some people recoil at the mention of spiritualism. Can you attend to your noös, or spirit, while adhering to a very scientific worldview? 

“Yes because the truth of one discipline can’t contradict the truth of another. There is absolutely no contradiction between the truths of revelatory faith, for instance, philosophical insights and scientific evidence. But Frankl uses the term ‘spirit’ in the classical German, Greek and philosophical sense without explicit reference to the religious register.

“In the old days we might have thought of spirit as somebody’s mettle, or internal power or resilience, something in them that’s more than them.”

The public health response to problems such as depression largely follows a medicalised model, and excludes this type of philosophical therapy. Is that a mistake?

“Absolutely, it is a category mistake. If you take depression, I mightn’t say there are three types of depression but if we can construe people in these three dimensions I have mentioned, then different responses are appropriate in each case.

“An appropriate response to an endogenous, biological or biochemical depression – such as bioplar – would be psychiatry or prescribed medication. But if you are depressed, let’s say, because you suffered the loss of a loved object, then it’s not biochemical; it’s psychological or reactive. Rather than medicalising that sadness a better course of treatment would be talking cures.

“Now, let’s say your depression is more philosophical, spiritual or existential, where you ask yourself ‘What is the point and purpose of human existence?’, that type of depression – if you want to call it that – would be noetic or spiritual, and meditation would be a better response than medication.

“So sometimes, to quote a famous bestseller, ‘Plato is better than Prozac’ but it depends on the symptoms, the syndromes, and what type of depression you are dealing with. Generally, they are all intermixed, muddied and muddled but one type of depression may be more in the ascendancy.”

There is very little public discussion about meaning in society today. Do you think that contributes to mental health problems, or inhibits human flourishing? 

“I do. I think we are witnessing at the moment the breakdown in traditional answers to questions of meaning. Previously we would go to our priest or politician. The traditional resources of church and state, however, have let people down. So what’s filling the gap? “What usually fills the void when you take away traditional values is sex and aggression. Or, you get boredom, where there is too little challenge; and anxiety, where the challenge is too much.

“To be mentally healthy is to achieve wholeness, which is the synthesis of these three parts of the human personality first laid out by Plato and retrieved for us by Frankl for our time.

“So it is very important that policy-makers, journalists, politicians, philosophers, get involved in a contemporary cultural debate in order to raise these questions to awareness and subsequently to action. We need philosophy in our secondary schools and to instigate talk about meaning and values, and to alert people’s attention to the presence of logotherapy.

“I know it saves lives because what people really want is a meaning to their personal existence, the most complete possible. Nothing less than that. Nothing more either.”

• Unthinkable will return in autumn
• Listen to Stephen J Costello at irishtimes.com. See also viktorfranklireland.com