Some Care With Words... [i.e., still, clear thinking - a contradiction in terms]
It is understood...
First, it is best that some care be taken with the words that one chooses... Otherwise, there is a more likelihood that some confusion will persist.
There is 'doing'… and then there is 'being'.
All thinking is a part of doing… whether it is positive or negative… all thinking is a doing.
Mind means movement... mind is always in flux... Tensions of body come out of movement of mind... And mind always moves... it moves into the past or into future… Hence mind means time.
This present moment is really not a part the horizontal plane of time running from past to future... the dimension of 'no time' runs on a vertical plane… it is right in between the two so-called 'tenses' of past and future. It is what makes up eternity...
You can move, more or less, deeper into 'this' present moment, or brush upon it very superficially... Moving deeper into the present is what meditation is all about. Zen Masters have called this the state ‘no mind”.
In this state, there is no thinking - positive, negative... like as if there is a cloudless sky... there is simply a clarity… a reflecting… of pure conscousness without any thought. this has nothing to do with mind itself… but is a consequence of putting the mind aside...
The phrase “peace of mind” is a misnomer. The same can be said for the phrase that you have used..."still, clear thinking"... This is also a contradiction of terms ... because thinking itself is a clutter...of noise...feelings...emotions...a collection of the past... and not consisting of any real clarity or stillness... the speed at which thoughts move can affect the amount of haze ...the amount of the thickness of the clouds upon the inner sky. But one can never experience a real peace through mind or with the mind. …it is not a possibility… the movement of the mind is what keeps peace from happening…
One may feel peaceful at one time or another without being aware of what is actually happening... this occurs because mind has somehow not gotten involved...peace comes, as if, almost by accident... not as a direct result of trying to make it happen. But comes, as if, through the back door of being involved with something that is unassociated...or unrelated. For an example, while driving your car and you make a turn and suddenly you see a beautiful sunset infront of you... looking at this beauty, it catches you off guard ...you may suddenly find yourself overflowing with a peacefulnness…
The reason is because mind... for a brief moment... is not involved with what is being witnessed... it has not yet begun its movement… you are being captured in an instant of awe and wonder... the mind is not engaged... this allows a sense of being to arise… the feeling of peace comes out of this… you are connected with 'being' and it has come without your expecting it to happen…
But as soon as you say to yourself “Oh my god, what a beautiful sunset!”, then the mind enters in... and begins moving... then peacefulness begins to fade... 'being', once again, becomes covered over by thought. One may remember the feeling that had come... but the actual experience of real peace is gone.
Or, one may find this... one may find that one feels more peaceful... or restful after taking a hot bath. While in the tub one is feeling the hot water against the skin of the body... feeling the heat penetrating. Making this a practice, it may become a form of meditation, when mind is put aside and it is not used to think about some future appointment or past event...
And so... relative to the health of the body… if one were to think... it is better to have positive thoughts rather than negative thoughts, because it would be easier to move from positive thought to no thought…rather than from negative thought to no thought… positive thoughts are lighter... negative thought is more burdensome more hard on the body...
There is no such thing as still, clear thinking.
So, if your not into meditation ... yeah sure think positive...it is better for health.