According to the sacred books & scriptures, God can only be seen when the eyes within are opened i.e. not by the physical eyes but by the Eyes of the heart'. The inner-self is the combination of mind, intelligence, heart & ego.
The invisibility of the God can be explained by the following example.
Let us suppose, vessel containing dirty water, which is also not still, is kept outside, under the Sun. A piece of cloth is hanging just above that vessel. In the mid-noon, when the Sun-God is just above the vessel, the image of it can not be seen in the water, because there are three obstacles in the visibility of the image. They are
Dirtiness of the water
Unstillness of the water
The obstacle caused by the piece of cloth
If all these three obstacles are removed then there is no reason why the image of the Sun-God can not be seen in the water. This can be done by purifying the water and making it remain still and by removing the piece of cloth from there.
Similarly there are three defects in a human being because of which the god remains invisible to him, They are the God remains invisible to him. They are :
Impurities caused by sinful actions
Restlessness of the Mind
Ignorance regarding unconscious & Consciousness and the inability to make discrimination between the two.
To remove out these three defects from a mans inner being is the only objective of the sacred Vedas. There are altogether one lakh mantras in all the four Vedas, Out of these, eighty thousand mantras deal only with the means to remove away the impurities from the inner self, by performing rituals & rites like yagya etc.
There are sixteen thousand mantras dealing with the means to control the restlessness and unsteadiness of mind and heart, by worshipping the images and idols of gods.
There are four thousand mantras dealing with the means to help make discrimination between Unconsciousness consciousness by the help of divine knowledge.
All these three categories of mantras have been divided into i) Karmakanda, ii) Upasana Kanda, iii) Gyankanda.
THE NECESSITY AND IMPORTANCE OF 'OLEANDER' (DURVAA GRASS, CHINA-ROSE, LEAVES OF BASIL PLANTS AND 'BAEL OR WOOD APPLE' IN THE RITUALS
There are many types of worship like SHODASH UPACHAARA (By making offerings of sixteen items) DASHOPACHAARA (ten items) & PANCHOPACHAARA (Five items).
Even in these various types of worships, there are specific offerings, which are considered as appropriate for one deity, but not appropriate for the another for example DOORVA GRASS is an ideal offering if mode to Lord Ganesha, the China-rose is an ideal offering if made to Sun god, The oleander flower is considered as an ideal offering if made to goddess Durga, Shiva loves bael, the most and Lord Vishnu accepts basil leaves with Joy.
A worshipper will have to follow the certain rules and procedures for the worship of any specific deity, for example to worship Lord Vishnu one needs a white coloured wollen seat, a beads of Basil wood, the pure flower Lotus, Saffron and white Sandal wood.
Similarly while worshipping Sun-god, red Sandal wood, and the China rose are a must.
If a person having a desire to get married wants to worship Goddess Durga then he must use a seat of Red wollen piece, Oleander flowers, currrent coconut fruit etc. Similarly for the worship of Lord Shiva, the leaves of Bael tree, thorn apple, Calotripis Rudraksha rosary etc.
Similarly for a worship of Lord Ganesha Durvaa Grass, Vermillion, and Modaka (Sweet) are required.
THE FOLLOWING SHOULD NOT BE OFFERED TO CERTAIN DEITIES.
Our Sacred texts and scriptures have looked with deep insight into the pros & cons of the offerings, which are made to the deities.
There are strict guideline as to what should be offered to the deity and what should be not. For example it has been prohibited to offer Akshata (grains or rice offered to deities during worship) to Lord Vishnu, the leaves of basil to Ganesha, Durvaa Grass to goddess Durga, and bael fruit to the Sun God.
The reason why leaves of the basil plant must be offered to Lord Vishnu is that Lord Vishnu rest on Sheshnaga in KsheerSoagara. The body of the Sheshnaga being cold Tulsi is considered as good offering, as it property is hot, Akshata (rice grains) having cold-property is also prohibited for the same reason.
Lord Shiva who is also known as Aashutosha had consumed poison, which was discovered while churning the ocean. This poison was not allowed to pass beyond the throat. The property of poison being very hot, the fruit of bael having cold property is considered ideal for him.
WHAT SHOULD BE OFFERED TO LORD GANESHA
Lod Ganesha who is also known as VRIKODARA which means who has fire by the name of VRIKA in his stomach i.e. one who has great hunger. So Durvaa Grass which has both cold and medicinal properties in it is considered as an ideal offering for Lord Ganesha.
Lord Ganesha, should not be offered such things which have hot properties, as it would magnigy the heat and hunger. Beside that Durvaa has also medicinal properties if the juice extracted from the Durva grasses are applied to any part of the body having a wound it cured the wound. The reason for offering Durvaa grass to Lord Ganesha is that, since Lord Shiva had cut Ganesha's head and an elephant head was fixed later on, So the heal the wound which still might not have healed, Durvaa grass is offered.