Education from ancient greece to early 20th century was largely a process of memorisation. Students were expected to learn, remember and use a large variety of cultural materials to furnish their thoughts, words and actions. Once finished with studies, students would continue to have poetry, literature, theatre, religion and history to draw on, wherever they were, when ruminating on particular struggles through their lives. Take Shakespeare, woven throughout his dialogues are poetic and cultural references and yet he was writing for a largely illiterate audience. These references may have helped people challenge their unhelpful thoughts.
I want to look at two things have occurred in the west, in the late 20th century in terms of their impact on mental health . The first is the trend in education to avoid memorisation and the second is the development and refinement of various cognitive behavioural therapies (CBT).
Since the education revolution of the 1970s, students in the west have largely stopped memorising. With the rise of the internet in the 1990s, this process has all but ceased, even for adults who were brought up believing that memorisation was an important aspect of living one's life. Advocates of the extended mind might say that access to the internet or books makes memorisation obsolete. But, think of driving one's car across town, or rock climbing or trying to cook a meal whilst managing anxiety and/or depression. When individuals are alone with their thoughts, when they cannot plug into the internet, or even when they can plug in, they can't necessarily bring to mind a reference or activity that would calm them, offer advice or solace to guide them back to a rational state of mind. Depressed people often turn to social applications such as FB to get help or to feel better, and can spend quite a deal of time there without any progress in their mental state at all. Even if a person does open a relevant page, they can find it difficult to concentrate or absorb external information in a psychopathic state of mind. I claim that the mnemonic structures found in religious texts, poetry and so forth used to form a buffer against one's own negative thoughts and no longer plays such a central role in people's daily mental health management. That is, there is something different about memorising and it could be the key to fixing depression. But, I'm not advocating a return to religion in order to get benefits.
I argue that the most important thing about memorising is that it makes it easier to resolve negative affect. When content is memorised it becomes effective self-talk, springing effortlessly to mind. Lack of energy, poor problem solving and reduced cognitive function are features of depression. I compare this process to learning self-defence by practising moves over and over again without threat, so that in the event of an actual attack, reactions are swift and effective. This leads me to CBT.
CBT is a set of methods of challenging unhelpful thoughts. It has been empirically shown  to have a large impact on "unipolar depression, generalized anxiety disorder, panic disorder with or without agoraphobia, social phobia, post traumatic stress disorder, and childhood depressive and anxiety disorders" . The techniques are varied and have been refined over decades, but the principles are clear. What improves depression is when patients actively acknowledge distorted thoughts, challenge them and/or observe them to lessen their impact. The process is very intense, confronting and requires discipline and perseverance to carry out. Part of the effort involved is absorbing and retaining the various 'reframes' of negative thought patterns into realistic, positive, yet believable statements--i.e. memorising them. Patients must begin by quite laboriously writing out their thoughts and analysing them. But, with time and practise those new thought patterns become dominant and reflexive. They have been memorised and are accessible, even during an 'attack'.
Much of the effectiveness of CBT is due to the benefit of memorisation, a skill known for thousands of years, but perhaps only recently rediscovered.
 I focus on the west in this case. But, clearly depression exists in Asian cultures and they have a very strong focus on memorisation. I should be very clear then in stating that I do not mean that memorising anything will help depression. But, that using memorisation with CBT (or perhaps religious texts, poetry etc…) is the combination required to ease symptoms.
 Butler, A.C., Chapman, J.E., Forman, E.M., Beck, A.T. (2006) The empirical status of cognitive-behavioral therapy: A review of meta-analyses Clinical Psychology Review 26(1) 17-31.