Definition of intelligence has been ambiguous till date. Closest definition of intelligence, most people would agree to, is the ability to acquire and apply knowledge and skills. Notion of seeing intelligence as single general ability was changed when Howard Gardner proposed theory of multiple intelligence in his book “Frames of Mind” in 1983. He differentiated intelligence into specific 'modalities'.
In his book, Gardner had described seven types of intelligence:
- Linguistic: ability to speak and write languages
- Bodily-kinesthetic: capacity to manipulate objects and use a variety of physical skills
- Interpersonal: ability to understand and interact effectively with others
- Intrapersonal: capacity to understand oneself and one’s thoughts and feelings, and to use such knowledge in planning and directing one’s life
- Spatial: ability to think or visualize in three dimension
- Musical: ability to discern pitch, rhythm, timbre, and tone
- Logical: ability to calculate, quantify, consider propositions and hypotheses, and carry out complete mathematical operations
In 1995, Gardner proposed one more type of intelligence:
8. Naturalist: ability to discriminate among living things (plants, animals) as well as sensitivity to other features of the natural world (clouds, rock configurations)
Today’s educationists, thinkers, reformers and leaders are stressing much on opportunities and ways to develop each of the above kinds of intelligence in students and youth for better life. But there is one more type of intelligence, leaving which aside man cannot best exercise any of the above intelligence to be happy and peaceful. This ninth type of intelligence is:
9.Spiritual intelligence: ability to question and understand the reason and purpose of existence. i.e. why we took birth in specific circumstances, why we suffer and enjoy, why do we die, what happens after we die, how universe came into being, why different species of life, how should we act, etc.
Modern science has rendered sufficient evidences that except spiritual intelligence all other eight types of intelligence are present in non-human species too. Among human beings too, spiritual intelligence is rarest of all type of intelligence. Not just rarest, it’s most important gift to human beings. Because by exercising spiritual intelligence only man can lead a meaningful, satisfactory and righteous life.
World without 9th type of intelligence:
In the modern society, man is giving more importance to work produced of first eight types of intelligence. This has resulted in 350 million people worldwide (i.e. 5% of world’s population) suffering from depression. This cannot be neglected, for depression is leading factor for increase in chronic diseases, anxiety, suicides, violent crimes, intoxication etc. When man is taxing one or more of eight types of intelligence to be happy, there are two things worth comparing between human and non-human beings:
- First, even though non-human species don’t possess advanced intelligence as much as human, we don’t find evidences of their suffering from depression in their natural condition of living.
- Second, man’s fundamental achievements by utilizing eight types of intelligence and that of animals are same: eating, sleeping, protection and enjoyment. In fact, animals have better facility for these four activities than average man of modern society.
World’s top tier universities have many departments to develop and apply each of eight types of intelligence to produce amenities for better living. But there is no department in any university, including MIT, to scientifically study the difference between living and dead body i.e. to study spirit. In other words there is no education to develop and apply spiritual intelligence for producing better living. This constitutes the fundamental reason for world’s worsening condition. In the name of secularism leaders of the modern world, having a less spiritual intelligence, have neglected the importance of practicing spirituality. It is different subject to discuss the difference between spirituality and religion, but important thing is to understand what is spirituality and how to practice it to lead better life.
Exercising the 9 th type of intelligence:
Unlike other type of intelligence, spiritual intelligence deals with spirit which is beyond gross elements (earth, water, air, fire & ether) and subtle elements (mind, eight type of intelligence & ego). It allows man to recognize that he is more than just a body, that he is soul. Not all being possess sufficient spiritual intelligence to understand and realize this seemingly simple fact. But just like eight type of material intelligence can be developed by training under the guidance of a teacher, spiritual intelligence can also be developed by training under the guidance of bonafide teacher.
Developed spiritual intelligence does not lead to sitting idly and not desiring or doing anything, as proposed by many so called spiritual leaders. Having developed spiritual intelligence one understands that he is not the body but the spirit soul which is part and parcel of supreme spirit.  Thus he starts desiring and acting according to that understanding. Such person need not give up any of his routine activities or profession but has to link and divert the result of all his activities toward the supreme spirit as guided by the bonafide spiritual teacher.  All authorized scriptures of the world recommends that both, the practice of developing spiritual intelligence and the process of linking with supreme spirit begins with the chanting of spiritual sound (mantra). Simplest and highest recommended mantra in majority of scriptures is: Hare Krishna Hare Krishna Krishna Krishna Hare Hare Hare Rama Hare Rama Rama Rama Hare Hare. 
 The elaborate knowledge of spirit and supreme spirit is explained in most
scientific spiritual book Srimad Bhagavatam. https://prabhupadabooks.com/sb
 Qualification of bonafide spiritual teacher are explained in various scriptures.