Mind – मनस्, has been referred to in Bhagavadgita, about 23-24 times. I will draw from these references, for building my Theory of Mind.

Mind is a Perception Organ in addition to the five  ज्ञानेन्द्रिय.’s.
मनःषष्ठानीन्द्रियाणि प्रकृतिस्थानि कर्षति  (१५-७);

Five Sense Organs of Knowledge (Jnanendriyas) and Manas together make the six instruments. Jiva drags them out from Prakriti, at the time of possessing a Body, and towards the Vishayas at the time of existence and then away from the body, at the time of leaving the Body.

In Vibhuti yoga, Sri Rayaru, first classifies Vibhuti, into two categories- viz. प्रत्यक्ष and तिरोहित; 
Rama, Krishna, Vedavyaasa etc are Pratyksha Vibhuti Rupas.

Then he  describes तिरोहित  hidden Vibhuti rupas, in three Dimensions: 

• Among Objects of similar vintage, Paramatma’s special presence is to be found in  the object which is “the best”; 
• Existing in all the Objects as their Niyamaka and 
• Deriving Etymology so that  that specific word conveys Paramaatmaa.

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In the Vibhuti yoga, Paramaatma describes himself as Manas.
इन्द्रियाणां मनश्चास्मि ।             Sri Rayaru gives the commentary as follows:           Manas is the instrument of perception (a form of knowledge);     Etymologically,  अवबोधरूपत्वात् मनः । “I am the mind because, its form is Knowledge” God’s form is Knowledge!!. He says.
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Manas is supposed to be superior to the five ज्ञानेन्द्रिय’s (which are superior to the five कर्मेन्द्रिय’s, in any case) and the objects of the ज्ञानेन्द्रिय’s. (विषय’s).
इन्द्रियेभ्यःपराण्याहुरिन्द्रियेभ्यश्च परं मनः। मनसस्तु परा बुद्धिर्यो बुद्धेः परतस्तु सः    ॥  (३-४२)
Sri Rayaru observes that “इन्द्रियेभ्यः परा ह्यर्था । अर्थेभ्यश्च परं मनः” इत्यध्याहारेण योजना ध्येया ॥“ In Kathopanishat we get this.
बुद्धि is superior to मनस् and परमात्मा is superior to बुद्धि. 
इन्द्रियाणि मनो बुद्धिरस्याधिष्ठानमुच्च्यते । 
Kaama- the real and main enemy of sensible behavior, is located in the five knowledge organs, Manas and Buddhi. Kama camouflages the true Knowledge and Rationality by pushing the person into a status of ignorance and perversion of thinking;
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Mind is like a lagaam- reins, in the tasks of managing this Ratha of the Body.  
Kathopanishad gives this allegory very categorically, in the following words; आत्मानं रथिनं विद्धि शरीरं रथमेव च।बुद्धिं तु सारथिं विद्धि मनः प्रग्रहमेव च ।।इन्द्रियाणि हयानाहुर्विषयांस्तेषु गोचरान् ।आत्मेन्द्रियमनोयुक्तं भोक्तेत्याहुर्मनीषिणः ॥ (१-३-३,४)

• The Jiva, pervading in the body, is the यजमान-charioteer; 
• this Body is the Chariot; 
• the Buddhi is the Driver (Saarathi); 
• Manas is the Reins; 
• all the Indriyas are the Horses; 
• their objects are the Destinations; 
• the Atmaa, composed of Indriyas and Manas , is the Enjoyer of all the Fruits;

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At several places, it is mentioned that Manas should be used to control the sense organs.मनः संयम्य मच्चित्तो युक्त आसीत मत्परः ।(६-१४);मनसैवेन्द्रियग्रामं विनियम्य समन्ततः ।। (६-२४)
यस्त्विन्द्रियाणि मनसा नियम्यारभतेऽर्जुन ॥ (३-७)
All the Indriyas need to be controlled through the Mind.
**********             Mind is very fickle.
चञ्चलं हि मनः कृष्ण प्रमाथि बलवद्दृढम् । तस्याहं निग्रहं मन्ये वायोरिव सुदुष्करम् ॥ (6-34).
Controlling Mind is as difficult as holding the wind under control; 
Wherever, the fickle mind wanders, it should be brought under control;
यतो यतो निश्चरति मनश्चञ्चलमस्थिरम् । ततस्ततो नियम्यैतदात्मन्येव वशं नयेत् । (6-26); 
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Peace and Composure of Mind are essential for happiness;मनःप्रसादः  सौम्यत्वं मौनमात्मविनिग्रहः। भावसंशुद्धिरित्येतत्तपो मानसमुच्यते ॥
Composure of Mind,  Mild disposition, Control  of the Self,, Purity of the Thoughts and the emotions- all this together constitutes मानस तपस् ।
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Positive Prescriptions for the Mind. There are many positive actions prescribed for the Mind.
दुःखेष्वनुद्विग्नमनाः (2-56);  Keep the Mind unperturbed during adversity; 
कायेन मनसा बुद्ध्या केवलैरिन्द्रियैरपि। 
योगिनः कर्म कुर्वन्ति सङ्गं त्यक्त्वाऽऽत्मशुद्धये ॥ (5-11); Yogis perform their Functions, for the purification of their self, through their body, their Minds,  buddhi and their pure Indriyas, by renouncing selfish motives; 
प्रशान्तमनसं ह्येनम्… (६-२७)  one having a Mind which is not attracted towards the ViShayas on its own;
मय्यासक्तमनाः पार्थ   (7-1);  One should engage the Mind in Paramaatma only;

मय्यर्पितमनोबुद्धिः (8-7)  Keep your mind engaged in me even up to the last minute;
मनो हृदि निरुद्ध्य च (8-12) At the time of final departure, keeping the Pranavayu attached to the hrunnaamaka Narayana ;
मय्येव मन आधत्स्व (12-8) Always focus your Mind on Me only;
मन्मना भव मद्भक्तः (18-65) Finally Keep your Mind in Me and Become My Bhakta….

This Cryptic Note could be elaborated to become a Big Thesis!!!!!!!!